What Is an Eternity Ring and When Do You Give One?

Getting the ring size right before ordering is one of the most practically important steps in buying any ring and one of the most commonly mishandled. This guide covers three methods for measuring ring size at home, ranked by accuracy, plus the five most common mistakes that produce incorrect measurements even when the method is followed correctly. The most accurate sizing is always taken in person at TJ Diamond's Auckland studio using calibrated metal sizing rings. At-home methods are good for planning and approximate sizing. For any ring intended for daily wear confirm the size in person before crafting begins. New Zealand Ring Sizing — The UK Letter System New Zealand uses the UK ring sizing system: letters rather than numbers. The most common NZ women's sizes are L to N (US 5.75 to 6.75). The most common NZ men's sizes are S to V (US 9.25 to 10.75). These are population averages  always measure the specific finger. NZ/UK Size US Size (approx) Circumference (mm) Diameter (mm) J 4.75 48.7 15.5 K 5.25 50.3 16.0 L 5.75 51.9 16.5 M 6.25 53.5 17.0 N 6.75 55.1 17.5 O 7.25 56.6 18.0 P 7.75 58.3 18.6 Q 8.25 59.9 19.1 R 8.75 61.4 19.6 S 9.25 63.0 20.1 T 9.75 64.6 20.6 U 10.25 66.2 21.1 V 10.75 67.8 21.6 W 11.25 69.4 22.1 Featured snippet target: "ring size chart NZ" — this table with NZ/UK letter, US size, circumference and diameter is what Google surfaces for this query. Method 1 — Paper Strip (Most Accurate) Cut a strip of paper approximately 1cm wide and 15cm long. Wrap snugly around the base of the correct finger tight enough not to slide over the knuckle, not so tight it creases the skin. Mark where the paper overlaps. Lay flat and measure the distance in millimetres. Find the millimetre circumference in the table above. That is your NZ/UK ring size. Measure in the evening fingers are at their largest in the evening and smallest in the morning. A morning measurement consistently produces a size too small. Method 2 — Existing Ring (Good Accuracy) Use a ring that fits the correct finger comfortably  on the same hand, same finger as the new ring. Place flat on paper, trace the inner circle. Measure the inner diameter in millimetres. Find the diameter in the table above. Note: ring size differs between the left and right ring finger. Always measure the specific hand and finger. Method 3 — String (Least Accurate) Wrap string around the finger and measure. String compresses under tension and gives a reading 1-3mm smaller than the actual circumference. If using this method, add 3mm to your measured length before looking up the size chart. Use as a rough starting point only. The Five Most Common Sizing Mistakes Measuring in the morning: Fingers are at their smallest. Produces a size too small. Measuring in cold temperatures: Cold contracts fingers. Measure at normal room temperature. Not checking knuckle clearance: The ring must pass over the knuckle. If the knuckle is significantly larger than the base of the finger, size to the knuckle. Measuring the wrong hand or finger: Left and right ring fingers are different sizes. Measure the exact finger the ring will be worn on. Taking only one measurement: Finger size fluctuates. Take three measurements at different times of day and use the middle value. Band Width and Sizing A wider band feels tighter than a narrow band at the same nominal size. If ordering a band wider than 4mm, consider sizing up half a size. TJ Diamond jewellers account for band width when taking measurements in person  this adjustment cannot be captured by circumference measurements alone. Surprise Proposals — Ring Size Without Measuring If a measurement is not possible before proposing, borrow a ring your partner wears on the ring finger of the left hand, trace the inner circle, and measure the diameter. Alternatively, ask a close family member. The most common NZ women's engagement ring size is N (approximately US 6.75)  but this is only an average. For any uncertainty, choose a plain solitaire or a half-set design that can be resized after the proposal. Browse our engagement rings and wedding bands  or book a studio sizing appointment at our Auckland studio before your ring is made. Our lifetime warranty includes one complimentary resize within the first 12 months for any TJ Diamond ring. If you are choosing a simpler design with maximum versatility, see our solitaire engagement rings collection  solitaires are the easiest engagement ring design to resize after a proposal. Related: What is an eternity ring?  Frequently Asked Questions   Q1: What ring sizing system does New Zealand use? New Zealand uses the UK ring sizing system, which expresses sizes as letters rather than numbers. The scale runs from A (smallest commercial size) upward, with half sizes expressed as a letter and a half (for example, M or M½). This differs from the US system (which uses whole and half numbers like size 6 or 6.5) and the European system (which uses millimetre circumference). When ordering from TJ Diamond, you can provide your size in NZ/UK letters, US numbers, or millimetre circumference — our team will confirm the correct conversion before your ring is crafted. Q2: What is the most common ring size for women in New Zealand? The most common NZ/UK ring sizes for women are L to N, which corresponds to approximately US sizes 5.75 to 6.75 and a finger circumference of approximately 51.9mm to 55.1mm. However, ring size varies considerably between individuals and is not reliably correlated with height, weight, hand size, or any other physical characteristic. Population averages are a starting point for a surprise purchase when no measurement is possible — they are not a substitute for actual measurement. For any ring intended for daily wear, TJ Diamond recommends measuring the specific finger in person at our Auckland studio. Q3: Can I measure my ring size at home without any tools? A rough estimate is possible without tools, but not a reliable measurement for a fine ring. Without tools, you can wrap a piece of string or a strip of paper around the finger and mark where it overlaps, then measure that length. The paper strip method is more accurate than string because string compresses under tension and consistently produces a measurement smaller than the actual finger circumference. For any measurement that will be used to craft a ring, particularly a full eternity band or a design that cannot be easily resized, TJ Diamond strongly recommends coming into our Auckland studio for professional sizing using calibrated metal sizing rings. Q4: Why do fingers change size throughout the day? Finger size fluctuates with temperature, time of day, recent exercise, fluid intake, and in some individuals, hormonal cycles. Fingers are typically at their smallest in the morning and at their largest in the evening, because warmth and circulatory activity throughout the day cause soft tissue to expand slightly. Cold temperatures cause fingers to contract; warm temperatures cause them to expand. The difference between a morning and evening measurement on the same finger can be up to one full ring size. For a consistent result, TJ Diamond recommends measuring in the evening at normal room temperature, ideally after light activity. Q5: Does ring width affect ring size? Yes, and this is one of the most commonly overlooked sizing factors. A wider band compresses more of the finger's surface than a narrow band, and as a result feels tighter at the same nominal size. A 7mm wedding band sized the same as a 2mm engagement ring band will feel noticeably tighter in wear. As a general guide, if you are ordering a band wider than 4mm, consider going up half a size from your standard measurement. TJ Diamond's jewellers account for band width in the sizing recommendation during Auckland studio appointments — this adjustment is not captured by circumference measurements alone. Q6: What happens if my ring does not fit after delivery? Plain gold and platinum bands can typically be resized up or down one to two sizes by a skilled jeweller without affecting the ring's integrity. TJ Diamond's lifetime warranty includes one complimentary resize within the first 12 months of purchase for any ring we make. Rings with diamonds set around the full circumference — full eternity bands, full pave rings — cannot be conventionally resized because the stone settings run through the section that would be added or removed. Channel-set and heavily embellished rings are more complex to resize than plain bands. For these styles, accurate pre-order sizing is essential. Q7: How do I find my ring size if I want to propose as a surprise? Several approaches work for a surprise proposal. If your partner owns a ring they wear on the ring finger of their left hand (the most common engagement ring finger), borrow it temporarily and trace the inner circle, then measure the diameter. Alternatively, ask a family member or close friend who may know the size. If no measurement is possible, the most common NZ women's engagement ring size is N (approximately US 6.75), but this is only an average. TJ Diamond recommends ordering a surprise engagement ring in a plain solitaire setting or a half-set design that can be resized, and bringing your partner in after the proposal for accurate sizing before the final ring style is confirmed. Q8: Is my ring finger the same size on both hands? Almost never. The ring finger on the left hand and the ring finger on the right hand are typically different sizes — often by half a size or more. This occurs because most people have a dominant hand that is slightly more developed through daily use, and dominant-hand fingers are often marginally larger. Always measure the specific finger on the specific hand the ring will be worn on. Do not assume that a size taken from your right ring finger applies to your left ring finger, or vice versa. If you are ordering a ring intended for the left hand, measure the left ring finger specifically.  

The bow-tie effect is a shadow pattern visible across the widest central section of certain elongated diamond shapes most commonly oval, pear, and marquise cuts. It appears as a dark horizontal area in the shape of a bow tie, running across the stone's midsection. In a well-cut diamond, it is subtle and adds depth. In a poorly cut stone, it is a dominant dark shadow that significantly detracts from the ring's visual appeal. The bow-tie effect does not appear on any GIA or IGI certificate. It cannot be assessed from photographs. It must be seen in person. This is the most important quality consideration for any elongated brilliant diamond, and the primary reason TJ Diamond assesses every oval, pear, and marquise diamond individually before setting. A diamond can carry a GIA Excellent certificate and a pronounced bow-tie shadow. The certificate does not disclose it. A product photograph will not show it. You need to see the stone in person. Why the Bow-Tie Happens In a round brilliant, the circular outline creates a symmetric facet arrangement where light entering from any direction is returned roughly equally from all parts of the stone. In an elongated shape oval, pear, marquise  the central facets are oriented differently from the facets at the rounded ends or pointed tips. In the central section, light reflecting off the pavilion facets exits through the sides of the stone rather than returning through the table to the observer's eye. This creates the characteristic dark shadow across the widest point. The size and intensity of the bow-tie depends on the diamond's specific pavilion facet angles in the central zone. A cutter who optimises the central angles can minimise the bow-tie. A cutter who maximises carat retention from the rough at the expense of proportional correctness produces a more pronounced shadow. Which Shapes Are Affected Oval diamond rings — the shape most significantly affected. The bow-tie falls directly across the most prominent visible area of the stone. See our oval engagement rings and our  oval diamond rings collections for our selection of individually assessed oval stones. Pear diamond rings — affected similarly to oval. The bow-tie typically appears lower in the stone, closer to the widest point. Wing symmetry significantly affects how evenly the shadow falls. See our pear engagement rings collection. Marquise diamond rings — both pointed tips converge, and the bow-tie appears across the widest central section. See our marquise engagement rings collection. Round brilliant, emerald, princess, radiant — not affected. Round brilliants have symmetric light return. Step-cut and princess/radiant facet arrangements do not produce this shadow pattern. Bow-Tie Severity — What Each Level Looks Like Severity What you see TJ Diamond stance None to minimal No visible shadow. Rare. Outstanding light return throughout. Select — exceptional quality Subtle Slight depth shadow in overhead light. Adds dimension. Considered desirable. Select — realistic optimum for most ovals Moderate Visible shadow in most conditions. Detracts from brilliance. Reject — below acceptable threshold Severe Dark butterfly immediately obvious. Dominates the stone. Reject — immediately apparent to all observers Why Photographs Cannot Show It Product photography uses a direct overhead light source — the exact condition that minimises bow-tie visibility. The same diamond that photographs brilliantly under studio lighting may display a pronounced shadow under the natural daylight or indoor lighting where it will actually be worn. Never assess an oval, pear, or marquise diamond from photographs alone. How to Assess It In Person View under multiple light sources. Move from direct overhead light to natural window light this reveals the real-world appearance. Tilt the stone side to side. A well-cut oval's bow-tie shifts with movement. A severe bow-tie maintains a fixed dark shadow regardless of angle. Compare two diamonds side by side. Bow-tie severity is much clearer in direct comparison than assessed in isolation. Ask whether the stone was individually assessed before purchase. TJ Diamond assesses every oval, pear, and marquise diamond for bow-tie severity at point of selection not from certificates. Is a Subtle Bow-Tie Desirable? Yes. A completely bow-tie-free oval is extremely rare and the cutting trade-offs required to achieve it often reduce other proportional qualities. Many experienced buyers and gemologists consider a subtle, well-contained bow-tie to add depth and dimension making the stone appear to have more internal volume than a completely flat-looking stone. The goal is not zero bow-tie. It is a bow-tie subtle enough to add depth without dominating the stone's appearance. TJ Diamond holds multiple oval and pear diamonds for side-by-side comparison at any studio consultation. Book a consultation to assess bow-tie in person. See our article on the best diamond cut for maximum sparkle for how the bow-tie fits into the wider picture of cut quality and light performance. Related: What is the best diamond cut for maximum sparkle? Frequently Asked Questions Q1: What is the bow-tie effect in a diamond? The bow-tie effect is a dark shadow pattern visible across the widest central section of certain fancy-shape diamonds — most commonly oval, pear, and marquise cuts. It appears as a dark horizontal area resembling a bow tie or butterfly shape, running across the stone's midsection and interrupting the diamond's brilliance in that region. The effect is caused by the optical geometry of elongated brilliant-cut diamonds, where light entering from above reflects off the central pavilion facets and exits through the sides of the stone rather than returning to the observer's eye. In a well-cut diamond, the bow-tie is subtle. In a poorly cut stone, it can be a dominant dark shadow. Q2: Does the bow-tie effect appear on a GIA or IGI certificate? No. The bow-tie effect does not appear on any GIA or IGI certificate. No diamond grading laboratory includes bow-tie severity in its grading reports because the effect is a viewing-condition-dependent optical characteristic rather than a measurable physical property of the stone. This means a diamond can have a pronounced bow-tie shadow and receive a GIA Excellent cut grade for its round dimensions (polish and symmetry) without the bow-tie being disclosed anywhere on the certificate. The bow-tie must be assessed visually, in person, under multiple light sources. This is one of the primary reasons TJ Diamond assesses every oval, pear, and marquise diamond individually before selection. Q3: Which diamond shapes are affected by the bow-tie effect? The bow-tie effect primarily affects elongated brilliant-cut diamond shapes: oval (most commonly and significantly affected), pear, and marquise. Elongated cushion cuts can also exhibit a similar shadow pattern in some cases. Shapes that are not affected include: round brilliant (symmetric light return from all directions prevents the shadow), emerald cut and Asscher cut (step-cut facets produce a different reflective pattern), princess cut (square brilliant facet arrangement does not produce the elongated shadow), and radiant cut (trimmed corners allow more even light distribution than strict elliptical outlines). Q4: Is some bow-tie effect desirable? Yes — a subtle bow-tie is considered desirable by many experienced diamond buyers and gemologists. A completely bow-tie-free oval diamond is technically possible but extremely rare, and the cutting required to achieve it typically involves trade-offs in other proportional qualities. A subtle, well-contained bow-tie creates an impression of depth and dimension, making the stone appear to have more internal volume than a completely flat-looking stone without any shadow contrast. The target for a well-cut oval, pear, or marquise diamond is not zero bow-tie — it is a bow-tie subtle enough to add depth without dominating the stone's appearance. TJ Diamond rejects diamonds with moderate or severe bow-ties, but selects those with subtle bow-tie depth. Q5: Can you see the bow-tie effect in product photographs? No — product photography almost always hides the bow-tie effect. Diamond ring photography uses a direct overhead light source positioned to maximise sparkle for the image. This specific lighting condition is precisely the one that minimises the bow-tie's visibility, because the overhead light directly illuminates the central facets that would otherwise be in shadow. A diamond can photograph as brilliantly sparkly under studio lighting and display a pronounced dark shadow under the office lighting or natural daylight where it will actually be worn. This is why viewing any oval, pear, or marquise diamond in person under multiple light conditions is essential before purchasing. Q6: How do I assess the bow-tie effect when buying an oval diamond ring? Three steps: First, view the diamond under multiple light sources — direct overhead light minimises the bow-tie; move the stone toward a window or under diffuse ambient light to see its real-world appearance. Second, tilt the stone slowly from side to side — a well-cut oval's bow-tie should shift and change as the stone moves, indicating good overall light return. A severe bow-tie maintains a fixed dark shadow regardless of movement. Third, compare two diamonds side by side under the same conditions — bow-tie severity is much easier to assess relatively than in isolation. TJ Diamond holds multiple oval and pear diamonds for comparison during Auckland studio consultations. Q7: Do oval diamond rings always have a bow-tie effect? Almost all oval diamonds have some degree of bow-tie effect — it is an optical characteristic inherent to the elongated brilliant cut geometry rather than a cutting flaw. The question is severity, not presence. The range runs from imperceptible (a very slight depth shadow that most observers would not notice and that adds dimension) to severe (a dark butterfly shadow immediately visible to any observer that significantly reduces the ring's visual appeal). TJ Diamond's selection process rejects stones with moderate or severe bow-ties and selects stones where the bow-tie is subtle or minimal. Our Auckland jewellers assess this for every oval, pear, and marquise diamond before it enters our workshop. Q8: Does the bow-tie affect an oval diamond's price? The bow-tie effect is not graded and does not appear on certificates, so it does not directly affect the wholesale price of a diamond. However, a well-cut oval with a minimal bow-tie commands a premium in the fine jewellery market because experienced buyers and jewellers know the quality difference. At TJ Diamond, we source ovals specifically for cut quality and bow-tie assessment — this means our selection skews toward better-cut stones, and our prices reflect the quality of the selection process rather than raw certificate grades alone. A cheaper oval diamond from a less selective source may have a more pronounced bow-tie at the same certificate grades.    

If you have ever compared two diamonds of the same carat weight and found a price difference of thousands of dollars, or looked at a 2 carat diamond and wondered why it costs four times as much as a 1 carat diamond rather than twice as much, this guide answers both questions. Diamond pricing is more structured and more counterintuitive than most buyers expect and understanding how it actually works is the single most useful thing you can do before buying a diamond ring. The short answer is this: diamond price does not scale linearly with carat weight. It scales exponentially, with significant price jumps at specific weight thresholds, and carat weight is only one of four quality variables that together determine the final price. A full understanding of how diamond price is calculated requires understanding all four of these variables, how they interact, and crucially at exactly which carat thresholds the price steps up sharply rather than gradually. A 2 carat diamond does not cost twice as much as a 1 carat diamond at the same quality grade. It typically costs three to five times as much. Diamond pricing is exponential, not linear and this is by design, not by accident. The Carat — What It Measures and Where It Comes From A carat is a unit of weight. One carat equals exactly 0.2 grams. One carat is subdivided into 100 points so a 0.50 carat diamond is also described as a 50-point diamond, and a 0.75 carat diamond as a 75-point stone. The carat system dates to antiquity, when merchants used carob seeds remarkably consistent in weight  as counterweights to measure gemstones. The word 'carat' derives directly from the Arabic word for carob. The modern carat was standardised internationally in 1907 at exactly 200 milligrams (0.2 grams). Before standardisation, carat weights varied between trading regions, creating significant inconsistency in the gem trade. The 1907 standardisation made carat weight the universal, objective measurement it is today the only fully objective variable in diamond grading, because weight is a physical measurement that cannot be influenced by grading opinion. Why Diamond Price Is Exponential, Not Linear The exponential relationship between carat weight and price reflects two compounding factors: rarity and demand concentration. First, rarity. Larger diamonds are exponentially rarer than smaller ones. Diamond rough forms in the Earth's mantle under conditions of extreme heat and pressure over millions to billions of years. Larger diamond crystals form less frequently than smaller ones. Among the diamonds mined globally, stones large enough to yield a 1 carat polished diamond are already uncommon. Stones large enough to yield a 2 carat polished diamond are substantially rarer. Stones that yield 3, 4, or 5 carat polished diamonds are exponentially rarer still — not simply twice or three times as rare, but orders of magnitude rarer. Second, demand concentration at round number thresholds. Buyers globally fixate on specific carat weights: 0.5 carat, 0.75 carat, 1.0 carat, 1.5 carat, 2.0 carat. A diamond just above these thresholds commands a meaningfully higher price than a diamond just below it, even when the face-up size difference is imperceptible. This demand concentration creates what the industry calls 'magic sizes' price steps that represent the most significant per-carat jumps in the market. The Rapaport Price List — How the Diamond Industry Sets Base Prices The foundation of global diamond pricing is the Rapaport Diamond Report, published weekly by Rapaport Group since 1978. The Rapaport list sets baseline prices for round brilliant diamonds in grids organised by colour grade, clarity grade, and carat weight. All other diamond trade between cutters, wholesalers, dealers, and retailers uses the Rapaport price as a reference point, with actual transactions occurring at a percentage above or below 'Rap' depending on market conditions and stone quality. The Rapaport structure makes the exponential price relationship between carat weights explicit and systematic. The price per carat for a 1.00-1.49 carat diamond at a given quality grade is set at a specific level. The price per carat for a 1.50-1.99 carat diamond at the same quality grade is set at a significantly higher level — not because the quality has changed, but because the supply of diamonds in that weight range is meaningfully smaller. This pricing structure means that crossing a carat weight threshold going from 0.99 carats to 1.00 carat, for example jumps the diamond into a higher Rapaport price bracket for its entire weight. The additional 0.01 carat is not what you are paying for. You are paying for the category threshold. The Rapaport price list is why a 0.99 carat diamond costs meaningfully less than a 1.00 carat diamond of identical quality. The 0.01 carat difference in weight triggers a category threshold. You are buying a diamond just below the magic size, not a lower-quality stone. The Magic Size Thresholds — Where Prices Jump Five carat thresholds create the most significant price steps in the consumer diamond market. Understanding them is the most practical application of diamond pricing knowledge for any buyer: Threshold Price per carat step Face-up size (round) Buyer strategy 0.50 ct (50 pts) Significant jump crossing 0.50 ~5.2mm Buy at 0.45-0.48ct for near-identical appearance, 15-20% less 0.75 ct (75 pts) Moderate jump crossing 0.75 ~5.9mm Buy at 0.70-0.73ct — imperceptible size difference, meaningful saving 1.00 ct — LARGEST JUMP Most significant in consumer market ~6.4mm Buy at 0.90-0.98ct — reads the same, 20-30% price advantage 1.50 ct Major jump crossing 1.50 ~7.4mm Buy at 1.40-1.48ct for near-identical face-up, meaningful saving 2.00 ct Largest premium tier ~8.2mm Buy at 1.85-1.95ct — the per-carat jump at 2.00ct is extreme The 1.00 carat threshold is the most commercially significant. A 0.95 carat round brilliant and a 1.05 carat round brilliant differ in face-up diameter by less than 0.15mm  imperceptible in any real-world viewing condition. The price difference at the same quality grade is typically 15-25%. Buying just below the 1.00 carat threshold is the single most straightforward budget strategy available to any diamond ring buyer. How Carat Weight Interacts With the Other Three Cs Carat weight determines which Rapaport price bracket a diamond sits in, but the three other grading criteria cut, colour, and clarity determine where within that bracket the diamond is priced. Understanding how these interact is essential to reading any diamond price correctly. Cut — The Multiplier That Affects Every Other Variable Cut quality is the single most important quality variable for a round brilliant diamond, and the one with the most direct impact on price at a given carat weight. A GIA Excellent cut diamond at 1.00 carat in G colour and VS2 clarity commands a meaningfully higher price than the same carat weight, colour, and clarity in a GIA Fair cut, because the Excellent cut returns significantly more light and is in genuinely higher demand. For round brilliant diamonds, GIA grades cut on a formal scale from Excellent to Poor. TJ Diamond recommends GIA Excellent or Very Good cut as the minimum at any carat weight below Very Good, the visual performance difference becomes apparent even without gemological training. Prioritising cut above colour and clarity at the same carat weight will consistently deliver a more brilliant ring at the same or lower price. Colour — The Variable Most Affected by Metal Choice Diamond colour grades run from D (perfectly colourless) to Z (visibly warm). At any given carat weight, moving from D to G colour at the same cut and clarity represents a saving of approximately 20-35% depending on the specific grade combination. Moving from D to H or I colour extends this saving to 35-50%. The practical value of colour grade selection depends entirely on the setting metal. In a platinum or white gold setting, a diamond at H or I colour may show a faint warmth visible to a trained eye. In a yellow gold or rose gold setting, the warm metal absorbs and neutralises the diamond's warmth G, H, and I colour diamonds read as colourless. Yellow or rose gold buyers can routinely select one or two colour grades lower than white metal buyers and achieve an indistinguishable visual result, saving significantly at any carat weight. Clarity — When Inclusions Are Visible and When They Are Not Diamond clarity grades run from Flawless (no inclusions visible under 10x magnification) to I3 (inclusions visible to the naked eye). At larger carat weights, inclusions become slightly more visible because the stone's larger face-up area can make internally visible features easier to spot at normal viewing distances. This means the minimum recommended clarity grade shifts somewhat with carat weight: 0.5-0.7 carat: SI1 with good clarity characteristics is typically eye-clean inclusions are not visible without magnification. 0.8-1.2 carat: VS2 is the comfortable minimum for a consistently eye-clean result. SI1 can still work if the specific inclusions are positioned toward the edge rather than under the table facet. 1.5 carat and above: VS2 or VS1 is recommended, because the larger face-up area makes SI1 inclusions more likely to be noticeable under normal viewing conditions. Emerald and Asscher cuts at any carat weight: VS2 minimum required, VS1 or above preferred. Step-cut facets act as transparent windows that reveal inclusions hidden in brilliant cuts. The Price Multiplier Table — How Much Does Each Additional Carat Cost? The following table shows the approximate price multiplier relationship between carat weights for natural round brilliant diamonds at G colour, VS2 clarity, GIA Excellent cut. These multipliers reflect the exponential, threshold-driven pricing structure of the diamond market: Carat weight Approx NZD range Price per carat vs 0.5ct (×) 0.50 ct $2,200–$4,500 $4,400–$9,000 per ct 1.0× 0.70 ct $3,500–$6,500 $5,000–$9,300 per ct ~1.2× 1.00 ct $5,500–$12,000 $5,500–$12,000 per ct ~1.6× 1.50 ct $12,000–$22,000 $8,000–$14,700 per ct ~2.4× 2.00 ct $22,000–$40,000+ $11,000–$20,000 per ct ~3.5× 3.00 ct $50,000–$90,000+ $16,700–$30,000 per ct ~6× G colour, VS2 clarity, GIA Excellent cut, 18ct gold solitaire setting. Indicative NZD retail ranges April 2025. Actual price depends on specific grade combination and setting. The multiplier column makes the exponential relationship explicit. Moving from 0.5 carat to 1.0 carat doubles the stone's weight but increases the price by 1.6 times (at the per-carat level). Moving from 0.5 carat to 2.0 carat quadruples the weight but increases the price by 3.5 times at per-carat rates and the total price increases by approximately seven to nine times. At 3 carats, the multiplier reaches six times the 0.5 carat per-carat rate. How Shape Affects the Price-Per-Carat Calculation The pricing structure above applies to round brilliant diamonds. Fancy shapes oval, cushion, pear, emerald, princess, radiant have their own price-per-carat levels, consistently lower than round brilliants at equivalent quality grades. The reasons are manufacturing-related: round brilliants require the most rough diamond waste of any cut (up to 60% of the original crystal), while fancy shapes follow the natural octahedral crystal form more closely, wasting less material. Round brilliant premium: 20-40% higher per carat than most fancy shapes at equivalent quality. The highest per-carat price of any common diamond shape. Oval, pear, marquise: 10-25% less per carat than round at equivalent quality. Oval delivers additional value through its face-up size advantage a 1 carat oval measures approximately 8×5.5mm versus 6.4mm for a 1 carat round, reading as visually larger on the hand. Cushion, radiant, princess: 15-30% less per carat than round. Square shapes with high sparkle. Princess cut corners require V-prong protection but otherwise carry no additional maintenance consideration. Emerald, Asscher (step-cuts): 15-25% less per carat than round, but require higher clarity grades (VS2 minimum). The clarity premium partly offsets the shape discount at the highest quality grades. The interaction between shape discount and magic size threshold creates the most powerful buying opportunity in the NZ diamond market: an oval at 0.95 carats in G colour, VS2 clarity reads as a 1.15-1.25 carat round on the hand (due to face-up size advantage), costs 20-25% less per carat than a round (shape discount), and sits below the 1.00 carat threshold (avoiding the threshold premium). The combined effect can represent 40-50% less in total spend for a ring that reads as larger than an equivalent-budget round brilliant solitaire. Lab-Grown Diamonds and Carat Weight Pricing The magic size thresholds and exponential price relationships described above apply primarily to natural diamonds. Lab-grown diamond pricing follows a similar exponential structure but at 50-70% lower price levels across all carat weights, and the threshold premium at magic sizes is less pronounced because the supply of lab-grown diamonds is less constrained by natural rarity. Practically, this means the 1.00 carat threshold jump that is so significant for natural diamonds is less acute for lab-grown  the percentage premium for crossing a threshold in lab-grown is typically smaller than for natural. This makes the strategic sub-threshold buying approach less financially critical for lab-grown buyers, though it is still worth considering. More importantly, lab-grown pricing creates an entirely different budget calculation. At $5,000-$6,000 NZD approximately the natural diamond price for a 1.00 carat round in G, VS2, 18ct gold a lab-grown buyer can access a 1.7-2.0 carat round at equivalent quality grades, with the exponential price increase for the natural stone not applying in the same way to the lab-grown equivalent. The Five Strategies That Follow From Understanding Diamond Price Calculation Understanding how diamond price is calculated leads directly to five practical strategies that most NZ buyers are not told before they start shopping: Buy just below magic size thresholds. A 0.90-0.98 carat diamond in excellent cut quality reads identically to a 1.00-1.05 carat stone on the hand. The face-up diameter difference is under 0.15mm. The price saving at the same quality grade is 15-25%. Prioritise cut above colour and clarity. A 1.00 carat diamond in GIA Excellent cut at G, VS2 delivers more visual performance than a 1.00 carat in Fair cut at D, IF. Cut is the variable that most directly determines what you actually see. Choose yellow or rose gold to unlock colour grade savings. At the same carat weight, selecting H or I colour in a yellow gold setting versus G colour in a white metal setting saves 15-25% with no visible difference in the finished ring. Consider the oval over the round for the same carat weight. An oval reads 10-15% larger on the hand than a round of the same carat, costs 10-25% less per carat, and benefits from the same magic size threshold strategies. Use lab-grown at the 1.5-2.0 carat threshold. The natural diamond exponential pricing is most severe at the larger carat thresholds. At 1.5 carats, a natural round in G, VS2 costs approximately $12,000-$22,000 NZD. The lab-grown equivalent is $3,000-$6,000 NZD. The exponential rarity premium for natural diamonds at this weight is where lab-grown delivers its largest absolute saving. Applying These Principles at TJ Diamond At TJ Diamond, every diamond ring price is calculated on the actual cost of the specific diamond sourced at the current market rate plus the craftsmanship of the Auckland studio setting. We do not add retail markup layers between you and the wholesale diamond market. When you book a studio consultation, our jewellers will show you specific diamonds at the carat weights you are considering, with the certificates, and compare them directly a 0.95 carat and a 1.05 carat round at the same quality grade, for example, so you can see and feel the difference (or lack of difference) in person before deciding whether the threshold premium is worth it for your specific ring. We also specifically compare fancy shapes against rounds at the same carat weight and budget point so you can see the oval's face-up size advantage concretely, not just read about it. Understanding how diamond price is calculated puts you in a fundamentally stronger position as a buyer. You will be able to identify where a price is justified by rarity and quality, and where it reflects a threshold premium or a shape premium that a small adjustment in specification would eliminate.