Stone to Carat Converter

1. How many milligrams are in a carat?

There are exactly 200 milligrams (mg) in 1 carat (ct). This precise definition was adopted internationally in 1907 at the Fourth General Conference on Weights and Measures in Paris and has been the global standard ever since. The 200 mg figure was chosen because it was close to the average of various historical carat standards (which ranged from 188-215 mg across different regions) and because it creates a clean metric conversion: 5 carats = 1 gram. Modern gemological laboratories use precision electronic scales that can measure to 0.001 carat (0.2 mg) accuracy, ensuring consistent and transparent gem trade worldwide. For comparison, a US dollar bill weighs approximately 1 gram = 5 carats, and a paperclip weighs ~1 gram = 5 carats. The 200 mg standard is legally enforced by metrology institutes in all countries, making carat weight the most objective and reliable of the "Four Cs" of diamond grading.

2. How many grams are in a carat?

There are exactly 0.2 grams (g) in 1 carat (ct), or conversely, 5 carats = 1 gram. This makes conversion straightforward: multiply carats by 0.2 to get grams, or multiply grams by 5 to get carats. Example: A 2.5-carat diamond weighs 2.5 × 0.2 = 0.5 grams (500 mg). The famous Hope Diamond (45.52 carats) weighs 45.52 × 0.2 = 9.104 grams. This clean metric relationship (5:1 ratio) was one reason the 200 mg standard was adopted in 1907—it integrates seamlessly with the metric system used globally in science and commerce. For context, a single M&M candy weighs approximately 1 gram = 5 carats, and a US nickel (5-cent coin) weighs 5 grams = 25 carats. The simplicity of the 5:1 carat-to-gram ratio facilitates international gem trade calculations and customs declarations.

3. Is carat the same as karat?

No, carat (ct) and karat (K or kt) are completely different units: Carat (ct) measures gemstone weight—1 ct = 200 mg. Used for diamonds, rubies, sapphires, emeralds, pearls, etc. Karat (K) measures gold purity—24K = 100% pure gold, 18K = 75% gold, 14K = 58.3% gold, etc. The fraction of pure gold is: (karat value ÷ 24). Example: 18K gold = 18/24 = 0.75 = 75% gold, 25% other metals (copper, silver, zinc for strength/color). Etymology: Both derive from the same root—"keration" (Greek for carob seed)—but diverged in meaning. In the US, different spellings distinguish them (carat/karat). In UK/Commonwealth, "carat" is used for both, distinguished by context and symbol (ct vs. K). Common example: "18-karat white gold engagement ring with a 1.5-carat diamond" correctly uses both units. Mixing them up causes confusion: "18-carat gold" is meaningless (gold purity isn't measured in gemstone weight units).

4. Why do diamonds cost more per carat as size increases?

Diamond prices exhibit exponential scaling—larger diamonds are disproportionately rarer, so price per carat increases with size. This is called the "carat premium." Example pricing (excellent cut, G color, VS2 clarity): 0.50 ct: $2,000 total = $4,000/ct. 1.00 ct: $6,000 total = $6,000/ct (50% higher per-carat than 0.5 ct). 2.00 ct: $22,000 total = $11,000/ct (83% higher per-carat than 1 ct, 175% higher than 0.5 ct). Why? Rarity: Only ~1 in 1,000 rough diamonds yields a 1-carat polished diamond; only ~1 in 1 million yields 2+ carats. Mining yields mostly small stones. Psychological milestones: Whole carat sizes (1.0, 2.0, 3.0) are culturally significant, driving demand and premiums. Implication: A 2-ct diamond costs ~3-4× more than a 1-ct diamond of identical quality, not 2×. Conversely, buying slightly below milestones (0.90 ct instead of 1.0 ct) can save 15-20% with negligible visual difference.

5. What's the largest diamond ever found?

The largest gem-quality rough diamond ever discovered is the Cullinan Diamond, found on January 26, 1905, at the Premier Mine in Cullinan, South Africa. It weighed an astonishing 3,106.75 carats (621.35 grams, about 1.37 pounds)—roughly the size of a human fist. The diamond was presented to King Edward VII of the United Kingdom and was subsequently cut by Joseph Asscher & Co. in Amsterdam into 9 major gems and 96 smaller brilliants: Cullinan I ("Great Star of Africa"): 530.2 carats, the largest clear-cut diamond in the world, set in the British Royal Sceptre. Cullinan II ("Second Star of Africa"): 317.4 carats, set in the British Imperial State Crown. Both are part of the British Crown Jewels displayed in the Tower of London. Other large rough diamonds: Lesedi La Rona (2015, Botswana): 1,109 carats. Sewelô (2019, Botswana): 1,758 carats (second-largest ever). However, the largest cut and faceted diamond is the Golden Jubilee at 545.67 carats, owned by the Thai Royal Family.

6. Do all gemstones of the same carat weight look the same size?

No! Carat is weight, not size, and different gemstones have different densities (specific gravity), so the same carat weight yields different sizes: 1-carat diamond (SG 3.52): ~6.5 mm diameter (round brilliant). 1-carat ruby/sapphire (SG ~4.0, denser than diamond): ~6.0 mm diameter—looks smaller than diamond. 1-carat emerald (SG ~2.71, less dense than diamond): ~6.9 mm diameter—looks larger than diamond. 1-carat opal (SG ~2.15, much less dense): ~7.5 mm diameter—looks much larger. Implication for buyers: If you want a larger-looking stone for the same budget, emeralds, aquamarines, or opals provide more "visual carat" than diamonds. However, diamonds are harder (Mohs 10) and more durable for daily wear. Cut also matters: A shallow-cut diamond may "spread" to look larger but sacrifices brilliance. A deep-cut diamond looks smaller but may have better light performance. Conclusion: Carat weight alone doesn't determine visual size—gemstone density and cut proportions both matter.

7. Are lab-grown diamonds measured in carats?

Yes, lab-grown (synthetic) diamonds are measured in carats identically to natural diamonds. They are chemically, physically, and optically identical to natural diamonds—pure crystalline carbon (C) with the same hardness (Mohs 10), refractive index (2.42), and density (3.52 g/cm³). Gemological laboratories (GIA, IGI) grade lab-grown diamonds using the same Four Cs (carat, cut, color, clarity) and issue certificates specifying exact carat weight. Key differences: Origin: Lab-grown created in controlled environments (HPHT or CVD methods) in weeks; natural formed over billions of years deep in Earth's mantle. Price: Lab-grown cost 30-70% less than natural diamonds of equivalent quality (e.g., 1 ct lab-grown ~$1,500-$3,000; natural ~$5,000-$8,000). Disclosure: Sellers must disclose lab-grown status; GIA certificates clearly state "laboratory-grown." Market: Growing acceptance, especially among younger, budget-conscious, or environmentally-minded buyers. Some prefer natural for rarity/tradition; others prefer lab-grown for value/ethics. Carat weight identical: 1 carat lab-grown = 200 mg, same as natural.

8. How accurate are carat weight measurements?

Modern carat weight measurements are extremely accurate, thanks to precision electronic scales: Precision: Gemological laboratories use scales calibrated to ±0.001 carat (±0.2 mg), often better. For reference, that's ~1/5th the weight of a single grain of table salt. Calibration: Scales are regularly calibrated against traceable reference standards linked to the SI kilogram (maintained by the International Bureau of Weights and Measures). Environmental control: Measurements performed in temperature-controlled rooms (~20-25°C) to prevent thermal expansion affecting readings. Cleaning: Gemstones cleaned before weighing (dirt, oils, or dust add weight). Repeatability: Multiple measurements averaged to ensure consistency. Certification: GIA, IGI, AGS certificates report carat weight to 0.01 ct (1 point) for consistency: e.g., "1.27 carats," measured to ±0.005 ct. Legal enforcement: Selling gemstones with misrepresented carat weight is fraud in most jurisdictions, punishable by fines and criminal penalties. Result: Carat weight is the most objective, precise, and reproducible of the Four Cs—far more consistent than subjective cut, color, or clarity grading.

9. What does "points" mean in diamond terminology?

In gem trade, 1 carat = 100 points, so 1 point = 0.01 carat = 2 milligrams. "Points" terminology allows precise description of smaller gemstones and fractional carats: Common examples: "50-pointer" = 0.50 carat (50 points). "75-point diamond" = 0.75 carat. "10-point melee" = 0.10 carat (small accent stones). "One-pointer" = 0.01 carat (tiny chip diamonds used in pavé settings). Why use points? Easier for small stones: Saying "ten-pointer" is quicker than "zero-point-one-carat." Pricing precision: Allows price-per-point calculations for wholesale. Trade jargon: Professional jewelers, cutters, and traders use points routinely. Historical: Predates electronic scales; points allowed finer distinctions than early balance scales. Example in retail: "This ring features a 75-point center diamond surrounded by 20 points total weight of accent stones" = 0.75 ct + 0.20 ct = 0.95 ct total carat weight (TCW). Note: While points are common in trade, consumer-facing marketing usually uses decimals: "0.50 ct" rather than "50 points."

10. Why did ancient people use carob seeds to weigh gems?

Ancient Mediterranean merchants used carob seeds (Ceratonia siliqua) as counterweights on balance scales because they believed these seeds had remarkably uniform weight—approximately 200 milligrams each, close to today's carat standard. Practical advantages: Abundance: Carob trees grew widely across Mediterranean (Greece, Italy, Middle East), making seeds readily available. Portability: Small, lightweight seeds easy to carry in pouches for trade. Durability: Dried carob seeds don't decay quickly; can be stored for years. Size: Perfect for weighing small, valuable items (gems, gold, spices, medicines). Ancient trade: Greek, Roman, Arab, and Persian merchants traveled long distances with balance scales and carob seeds, facilitating gem trade across continents. The belief in uniformity: Ancient texts suggest carob seeds were thought to have nearly identical weights, making them natural standard units. Reality: Modern analysis shows carob seeds actually vary by 10-20% (180-220 mg), so ancient weights weren't as precise as believed. However, the average (~200 mg) happens to closely match the modern carat, suggesting ancient merchants intuitively converged on a practical standard that persists today.

11. Can you haggle over carat weight when buying diamonds?

No, carat weight is objectively measurable and non-negotiable—a 1.00 ct diamond weighs exactly 200 mg, confirmed by precision scales. You cannot "haggle" to get 1.10 ct weight for the price of 1.00 ct. What you CAN negotiate: Price per carat: Total cost for the specific diamond (negotiating discount on asking price). Quality trade-offs: Choosing lower color (G vs. D), lower clarity (VS2 vs. VVS1), or different cut to stay within budget while maximizing carat. Just-below milestones: Purchasing 0.90 ct instead of 1.00 ct saves 15-20% with negligible visual difference—smart buying strategy, not haggling on weight. Total jewelry piece: Negotiating on the setting, band material, or package deal (ring + earrings). Trade-in: Bringing old jewelry to trade toward purchase. Timing: Shopping during sales (Black Friday, Valentine's Day) or at end of month/quarter when sellers have targets. What sellers CANNOT do: Misrepresent carat weight. Gemological certificates from GIA/IGI specify exact weight. If a seller claims a 1.00 ct diamond weighs more/less than 200 mg, it's either fraud or an error. Pro tip: Focus negotiations on price-per-carat reduction or value-adds (free resizing, upgraded setting), not on changing the actual diamond weight.

12. How has the carat system changed with modern technology?

The carat definition (200 mg) remains unchanged since 1907, but technology has revolutionized measurement, cutting, and trade: Precision scales: Modern electronic scales measure to ±0.001 ct (±0.2 mg) vs. early 20th-century balance scales (±0.05-0.10 ct). Ensures consistent global trade. Laser cutting: Computer-controlled lasers cut diamonds with micron-level precision, optimizing yield from rough and maximizing brilliance for a given carat weight. Can achieve ideal proportions consistently. 3D scanning: Machines scan rough diamonds in seconds, calculating optimal cut to maximize carat weight retention while achieving excellent proportions. Before: Cutters relied on experience; now: Algorithms optimize. Synthetic diamonds: Lab-grown diamonds (HPHT, CVD) can be produced to target carat weights, unlike mining where sizes are random. Enables consistent supply of specific sizes. Online retail: Internet sellers (Blue Nile, James Allen) list thousands of diamonds with exact carat weights, photos, 360° videos. Buyers filter by precise carat ranges (1.00-1.25 ct). Blockchain tracking: Some companies use blockchain to track diamonds from mine to retail, recording carat weight at each stage, enhancing transparency. AI grading: Emerging AI systems assist gemologists in assessing cut quality and predicting optimal finished carat weight from rough. Result: The carat unit is stable, but technology makes measurement, cutting, and trading far more precise, efficient, and transparent than ever before.