Carat vs Grams: How Gemstones and Mineral Specimens Are Weighed
You're browsing two listings. One is a spinel at 3.46 ct. The other is a crystal cluster at 500 g. Which one is heavier?
The cluster, by a lot. Five hundred grams is 2,500 carats. But if you've never had to convert between the two, the 3.46 looks like the bigger number, and plenty of buyers have stumbled over exactly that.
The short answer: one carat equals 0.2 grams, so five carats make one gram. Cut stones and small crystals are usually weighed in carats. Mineral specimens are usually weighed in grams or kilograms. To go from grams to carats, multiply by 5. To go from carats to grams, divide by 5.
The rest of this guide covers where the carat came from, why both units show up in the same shop, why carat weight doesn't tell you how big a stone looks, and a quirk that only matters for raw specimens: the weight includes the rock the crystals grow on.
Carats and grams: the quick conversion
You can do this in your head once you've seen it a few times.
| Unit | Equals |
|---|---|
| 1 point | 0.01 carat, or 2 milligrams |
| 1 carat | 0.2 grams (200 milligrams) |
| 5 carats | 1 gram |
| 100 carats | 20 grams |
| About 142 carats | 1 ounce |
| 5,000 carats | 1 kilogram |
Some quick examples
- A 3.46 ct oval spinel weighs about 0.69 grams. That's less than a paperclip.
- A 7.00 ct emerald-cut sphene weighs about 1.4 grams.
- A 14.95 ct raw crystal weighs about 3 grams.
- A 586 g specimen is about 2,930 carats. Nobody would quote it that way, which is the whole point of having two units.
Why gems use carats and specimens use grams
Where the carat comes from
The story goes that the carat started with carob seeds, which merchants used as balance weights because the seeds are fairly uniform. It's a nice story, though historians say the details are fuzzy. What isn't fuzzy is the result. Different countries used slightly different carats for centuries, until the metric carat was adopted in 1907 and fixed at exactly 200 milligrams.
That small size is the reason the carat survived. Gems are tiny and valuable, and a weight that's a fifth of a gram lets sellers price a stone down to a hundredth of a carat.
Why raw specimens are weighed in grams and kilograms
A mineral specimen is a whole piece of rock with crystals growing on it, sometimes heavy, often irregular. Carats are too small a unit for that. Quoting a 5 kg aquamarine cluster as 25,000 carats would just be silly.
Grams and kilograms also match how most collectors think. They want to know what a piece weighs in the hand, and they want its dimensions in millimeters or centimeters.
When you'll see both
You'll see both units in a single shop, and the split is usually this:
- Faceted stones: carats, always.
- Small raw crystals and gem rough: often carats, because the piece is small and might be cut later.
- Larger crystals, clusters and matrix pieces: grams or kilograms.
- Polished pieces like tumbles and spheres: grams or kilograms.
If you see a listing that mixes things up, like a raw crystal in carats next to a cluster in grams, it isn't a mistake. They're just different sizes of object.
Carat vs karat
Quick note, because it trips people up. Carat (ct) measures the weight of a gemstone. Karat (K) measures the purity of gold, like 14K or 18K. They sound identical, they're spelled almost the same, and they have nothing to do with each other. A "2 ct ring in 18K gold" means a 2-carat stone in 18-karat gold.
Carat weight is not size
Here's where most shoppers get caught. A carat is a unit of weight. Two stones that weigh the same can look quite different in size, because different gems are different densities.
Why density changes size
Gemologists measure density as specific gravity (SG). The higher the number, the heavier a stone is for its size. Diamond is about 3.52. Sapphire and ruby are about 4.0, so a sapphire packs more weight into less volume and looks smaller than a diamond of the same carat weight.
Think of a baseball and a tennis ball that happen to weigh the same. They wouldn't, because the baseball is denser. Gems work the same way.
| Gem | Typical SG (approx.) | Weight of a 6.5 mm round stone, if a diamond of that size is 1.00 ct |
|---|---|---|
| Diamond | 3.52 | 1.00 ct |
| Sapphire / ruby | 4.0 | about 1.14 ct |
| Spinel | 3.6 | about 1.02 ct |
| Peridot | 3.3 | about 0.95 ct |
| Tourmaline | 3.06 | about 0.87 ct |
| Aquamarine | 2.7 | about 0.77 ct |
| Quartz / amethyst | 2.65 | about 0.75 ct |
| Opal | 2.15 | about 0.61 ct |
A worked example
Say you're comparing a 2 ct sapphire with a 2 ct aquamarine. The sapphire's SG is about 4.0 and the aquamarine's is about 2.7, so the sapphire is roughly 1.5 times denser. The aquamarine takes up about 1.5 times the volume. Because size across a stone grows with the cube root of volume, the aquamarine ends up about 14 percent wider.
You can do this yourself: divide one stone's SG by the other's, then take the cube root.
These numbers are approximate. Sources report slightly different sizes for a 1 ct sapphire, from about 6.0 to 6.2 mm, because cut depth and proportions change things too.
Cut matters too
Two stones with the same SG and the same weight can still look different. A deep, narrow cut hides weight in the base, so it looks smaller from the top. A shallow, wide cut spreads that same weight across the face, so it looks larger. That's why sellers should list millimeter dimensions alongside carat weight. Our guide to gemstone cuts and shapes shows how shape changes the way a stone sits and sparkles.
If you're choosing a ring stone, compare millimeters first, then weight. Browse stones with both listed in our loose gemstones collection, or see how stones like those in our spinel gems collection are listed in carats with their cuts.
Weighing mineral specimens: the matrix question
Specimens have a quirk that cut stones don't.
The weight includes the rock
A specimen's listed weight is the whole piece, including the matrix, the host rock the crystals grow on. A 500 g cluster might be 400 g of mica or feldspar with a few beautiful crystals on top. Those crystals could weigh almost nothing. That's normal, and it isn't a trick. The matrix is part of what makes the piece a natural specimen.
It does mean weight alone tells you very little about a specimen's value. A smaller piece with well-formed, gem-colored crystals can be worth far more than a larger one that's mostly rock.
What to look at instead of weight
Weight is one number. Better questions to ask:
- How big are the crystals themselves, not just the piece?
- Are the points intact, and are there any breaks or repairs?
- How good is the color and transparency?
- Does the piece sit well, or is it awkward and lopsided?
Our guides on how to evaluate the quality of a mineral specimen and what makes a specimen valuable go through these in detail.
Ask for dimensions and a size reference
A good listing gives length, width and height in millimeters or centimeters. A photo next to a coin or ruler helps even more. If a listing only gives weight, ask for dimensions before you buy. For a closer look at the difference between raw pieces and cut stones, our guide to mineral specimens vs gemstones covers how each is judged.
You can compare real matrix specimens in our aquamarine crystals collection, where many pieces pair blue crystals with mica or feldspar, or browse the full range in specimens and crystals.
How to read a listing
| You see | It usually means | Worth checking |
|---|---|---|
| 3.20 ct | The weight of a cut stone or small crystal | Size in millimeters |
| 586 g | The whole piece, matrix included | Dimensions and how much is crystal |
| 2.00 TCW | Total carat weight of all stones together | The weight of each stone |
| 18K | Gold purity, not weight | Nothing to do with the stone |
| 25 x 18 x 12 mm | Length, width and height | Whether it matches the photos |
Mistakes that cost buyers money
- Mixing up carat and karat. One measures stones, the other measures gold.
- Comparing price per carat across different gems. A carat of opal and a carat of sapphire aren't the same volume, so the comparison isn't fair.
- Treating weight as size. Always check the millimeter dimensions.
- Forgetting the matrix. A heavy specimen isn't automatically a better one.
- Forgetting that cutting loses weight. A rough stone often loses more than half its weight when it's cut and polished, which is why finished stones cost more per carat than rough.
If you're buying online, our guide to buying loose gemstones without getting scammed has a checklist for what to ask the seller.
Frequently asked questions
How many grams is 1 carat?
One carat is exactly 0.2 grams, or 200 milligrams. Five carats make one gram.
How do I convert grams to carats?
Multiply the weight in grams by 5. For example, 2 grams is 10 carats. To go the other way, divide carats by 5.
Is carat weight the same as size?
No. Carat is a unit of weight. Two stones of the same carat weight can look different in size because different gems have different densities, and because the cut changes how the weight is spread.
Why does a 1 carat sapphire look smaller than a 1 carat diamond?
Sapphire is denser than diamond, so the same weight takes up less space. A 1 ct sapphire is a little smaller across than a 1 ct diamond cut the same way.
What's the difference between carat and karat?
Carat (ct) measures the weight of gemstones. Karat (K) measures the purity of gold, such as 14K or 18K.
Are mineral specimens sold by the carat or by the gram?
Mostly by the gram or kilogram, because a specimen is a whole piece of rock. Small raw crystals are sometimes listed in carats, especially when they may be cut later.