Moissanite vs diamond: same size, different carat weight

The stone that grew

Switch a 1.00 carat T73 from diamond to moissanite in the simulator, and its measurements grow. You don't touch the weight. The loupe test is a test for your eyes. This page gives you a test for a scale and a gauge. It starts from one line of the property card: specific gravity.

Weight for size

Specific gravity (SG) compares a stone's weight with the weight of the same volume of water. Diamond is 3.52; moissanite is 3.22.

So a moissanite is lighter for its size. Two stones of exactly the same shape and size weigh in the ratio 3.22 to 3.52. So the moissanite weighs about 0.91 times as much as the diamond. GIA's 1997 study gave the same example. A 6.50 mm round diamond weighs about 1.00 carat. A moissanite of the same diameter and cut weighs about 0.91 carat.

Turn it around and the same thing happens in reverse: at the same carat weight, the moissanite is the bigger stone. Not by much. Weight goes with volume, so the width changes with the cube root of the ratio. A moissanite of the same weight is only about 3% wider.

A carat is a unit of weight, 0.2 g, not a unit of size. That's why the same "1 carat" can be two different sizes.

Try it yourself

1. Open 3D Gemstone Simulator with the T73 Round Brilliant in diamond at 1 carat. Read the Measurements line.
2. Switch the gemstone to moissanite. Read the measurements again.
3. Set 2 carats and repeat for both stones.
4. Work it out. What would a moissanite exactly the size of the 1.00 carat diamond weigh? (Multiply by 3.22 ÷ 3.52.)
5. Bonus: switch to cubic zirconia at 1 carat.

What the simulator shows

At 1.00 carat the T73 measures 6.48 mm across in diamond and 6.67 mm in moissanite. At 2.00 carats, 8.16 mm against 8.41 mm. Both times the moissanite is about 3% wider. A moissanite exactly 6.48 mm across would weigh about 0.91 carat.

The bonus shows why moissanite is harder to catch than cubic zirconia. CZ is far heavier, SG about 5.8, so 1.00 carat of it is only 5.48 mm across. A CZ sold by weight as a diamond looks small for its carat. A moissanite looks only slightly large. GIA described moissanite as much closer to diamond in "heft" than any earlier imitation.

Using it on a loose stone

For a loose round stone, measure the diameter with a gauge and weigh the stone on a carat scale. Is the weight low for the size, by roughly a tenth for a well-proportioned round? Then look harder.

It's a clue, not a verdict: proportions change weight too. A deep stone weighs more for its width than a shallow one, and a thick girdle adds hidden weight. So compare like with like: same shape, similar proportions. The check works much better on a stone with a report. There you can compare the stone with the measurements and weight on paper (see moissanite certificates vs diamond reports).

Labs and dealers that sort mixed parcels use a heavy liquid instead. In methylene iodide, SG 3.32, a moissanite floats and a diamond sinks. GIA called specific gravity diagnostic. The liquid is toxic, so this is a lab method, not a kitchen-table one.

Key points

• Moissanite's specific gravity is 3.22 against diamond's 3.52.
• At the same size, a moissanite weighs about 0.91 times as much as a diamond.
• At the same carat weight, a moissanite is about 3% wider: 6.67 mm against 6.48 mm for a 1.00 carat T73.
• Weight for size is a useful check on a loose stone. It works best when you compare it with a report or with stones of similar proportions.
• In a heavy liquid of SG 3.32, moissanite floats and diamond sinks.

Go further: the moissanite gemstone page answers "How much does moissanite weigh compared with diamond?" in one line.