How to Measure Ring Size at Home

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.    

An eternity ring is a band set with diamonds running either fully around the circumference or along the top half only. The unbroken circuit of stones no beginning, no end is both the design and the meaning: an enduring love, a lasting commitment, a milestone that has no natural stopping point. At TJ Diamond, eternity rings are among our most requested commissions after engagement rings. Full Eternity vs Half Eternity — The Decision That Matters Most Full eternity ring: Diamonds set continuously around the entire band. The most visually impressive configuration. Cannot be conventionally resized after setting accurate sizing before ordering is essential. Half eternity ring: Diamonds set along the top half only, with plain metal on the underside. Can be resized one to two sizes. From the front, indistinguishable from a full eternity in most viewing conditions. If you are ordering as a surprise gift, choose a half eternity. The plain metal underside allows conventional resizing after the ring is received. A full eternity ordered in the wrong size cannot be easily corrected. Setting Styles for Eternity Rings Channel setting: Each diamond held between two parallel metal rails. Smooth, flush, snag-free. The best daily-wear configuration. Most popular for eternity wedding bands. Prong setting: Each diamond held by individual claws. More light reaches the stone higher sparkle. Requires annual prong inspection. TJ Diamond lifetime warranty covers this at no charge. Pave setting: Very small diamonds set closely together with tiny metal beads. A continuous river of sparkle. Most luxurious appearance. Requires careful wear and regular inspection. Diamond Shapes in Eternity Rings Round brilliant diamonds — tile evenly around any band width for a seamless, gapless circuit of sparkle. See our full round engagement rings collection for our available round stone specifications. Princess cut diamonds — square outline tiles as cleanly as round. Geometric and contemporary character. Emerald cut eternity bands — three rectangular step-cut stones side by side create a striking architectural band. A deeply sophisticated alternative to brilliant cuts. When to Give an Eternity Ring Milestone anniversary — 5, 10, 20, 25, or 50 years. The most traditional occasion. The unbroken diamond circuit references the duration of the relationship. Birth of a first child. A circle of diamonds referencing new beginnings. A narrow pave eternity in rose gold is among our most popular configurations for this occasion. As a wedding band alongside an engagement ring — a full or half eternity creates a deeply impressive bridal combination. See our full wedding bands collection for all band styles. Milestone birthday or self-purchase. The unbroken circuit carries personal meaning equally well — a significant life chapter, an achievement, a new beginning. Metal Choice for Eternity Rings Platinum: platinum engagement rings never needs replating, work-hardens rather than eroding, most hypoallergenic. The most popular eternity ring metal at TJ Diamond. The dimensional integrity of the band setting is best preserved over decades in platinum. Yellow gold: yellow gold engagement rings also never needs replating. Provides the colour grade advantage (G-H colour diamonds appear as colourless in yellow gold). Strong return in popularity in NZ in 2024-25. Rose gold: warm, romantic, no replating required. Particularly popular for first-child eternity rings. Sizing — The Most Important Practical Note Full eternity rings cannot be conventionally resized. There is no plain metal section available for size adjustment. For full eternity rings, TJ Diamond takes the measurement in our Auckland studio using calibrated metal sizing rings, and accounts for band width  wider bands feel tighter and require a slightly larger size. For any uncertainty about sizing, choose a half eternity. Browse our full range of diamond wedding bands and eternity ring styles or contact us to discuss your eternity ring requirements and book a studio sizing appointment. Related: Engagement ring vs wedding ring Frequently Asked Questions   Q1: What is the difference between a full eternity ring and a half eternity ring? A full eternity ring has diamonds set continuously around the entire circumference of the band. A half eternity ring has diamonds set along approximately the top half only, with plain metal on the sides and underside. The most important practical difference is resizability: a half eternity can typically be resized one to two sizes because the plain metal sections allow conventional size adjustment. A full eternity cannot be conventionally resized after setting because the stone settings run through the section that would be added or removed. If you are ordering a full eternity ring, accurate sizing before the ring is made is essential. Q2: When is an eternity ring traditionally given? The most traditional occasion is a significant wedding anniversary — five years, ten years, twenty-five years, or fifty years. The unbroken circuit of diamonds references the duration of the relationship: no beginning, no end. Other increasingly common occasions in New Zealand include the birth of a first child, a milestone birthday, and as a wedding band alongside an engagement ring. There is no rule about when an eternity ring must be given. Any occasion that carries the meaning of enduring love or commitment is appropriate. Q3: Can an eternity ring be worn as a wedding band? Yes. An eternity ring worn as a wedding band at the ceremony is one of the most popular configurations at TJ Diamond. A full or half eternity in platinum or yellow gold worn beside or instead of a traditional plain band creates an impressive bridal combination. The most important consideration when wearing an eternity ring as a wedding band is that it must be sized precisely before ordering, because it cannot be easily resized after setting if it contains diamonds around the full circumference. Q4: What is the difference between a channel-set and prong-set eternity ring? In a channel-set eternity ring, each diamond is held between two parallel metal rails rather than by individual prongs. The result is a smooth, flush surface with no protruding elements — the most snag-free configuration for daily wear. In a prong-set eternity ring, each diamond has its own individual claws, typically four per stone. More diamond surface is exposed to light in a prong-set ring, producing slightly more brilliance and scintillation. Prong-set eternity rings require periodic prong inspection — TJ Diamond recommends a professional check every 12 months. Our lifetime warranty covers this prong maintenance at no charge. Q5: Can a full eternity ring be resized? A full eternity ring cannot be conventionally resized after setting because the stone settings run continuously around the entire band — there is no plain metal section to add or remove for a size adjustment. If a full eternity ring is the wrong size, the options are limited to: wearing it on a different finger, having the stones reset into a new band of the correct size (significant cost), or in some cases adding small ring sizers to the inside of the shank. This is why precise pre-order sizing at TJ Diamond's Auckland studio is so important for full eternity rings. If you are ordering as a surprise gift, a half eternity is strongly recommended. Q6: What metal is best for an eternity ring? Platinum is the most popular metal for eternity rings at TJ Diamond for three reasons: it never needs replating (white gold requires rhodium replating every 1-3 years), it work-hardens rather than eroding with daily wear so the band retains its dimensional integrity over decades, and it is the most hypoallergenic precious metal available. Yellow gold is the second most popular choice — it also never needs replating, provides a colour grade advantage for the diamonds it holds (G and H colour stones appear as colourless in yellow gold), and has seen a significant return in popularity in New Zealand in 2024-25. Q7: How much does an eternity ring cost in NZ? Eternity rings at TJ Diamond start from $999 NZD for a diamond-set half eternity band. The total price depends on: the total diamond carat weight across all stones, the diamond quality (colour and clarity), the setting style (channel, prong, or pave), the metal (yellow gold, rose gold, or platinum), and whether the ring is a full or half eternity. Lab-grown diamonds make eternity rings significantly more accessible — the same visual impact with 50-70% less spend on the diamond content. Contact us for a personalised quote based on your specific eternity ring requirements. Q8: What diamond shape works best in an eternity ring? Round brilliant diamonds are the most popular and most practical choice for eternity rings. Their circular outline tiles evenly around the band at any width, producing a continuous, gapless circuit of sparkle. Princess cut diamonds also tile well due to their square outline. Emerald cut eternity rings are a beautiful option for buyers who want the step-cut architectural character in a band format — three emerald cuts side by side create a striking geometric composition. Oval, cushion, and pear shapes can be used in eternity bands but require more careful spacing and produce a slightly less seamless result than round or princess cuts.    

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.