Why the Same Carat Weight Looks Different

Weight is volume times density

Every stone's weight is its volume multiplied by the density of its material. Gemologists express density as specific gravity (SG), meaning how many times heavier the material is than the same volume of water. If two stones weigh the same, the one with lower SG must have more volume, so it looks bigger.

From left to right the stones get lighter per cubic millimetre, so they get bigger. The CZ is about 5.4 mm across and the opal about 7.5 mm, for exactly the same weight. Our specific gravity chart lists the density of every material we support.

Factor 1: the material

Density is the biggest single factor when you compare different gems. Here is how three common comparisons work out with our engine:

• Sapphire vs diamond. Sapphire is denser. A sapphire cut to the size of a 1 ct diamond weighs about 1.14 ct. A 1 ct sapphire faces up about 4% smaller in diameter.
• Emerald vs diamond. Emerald is lighter. A 1 ct emerald has the volume of a 1.29 ct diamond.
• Moissanite vs diamond. Moissanite is a little lighter. Cut to the size of a 1 ct diamond, it weighs about 0.91 ct, which is why it is usually sold by diamond equivalent weight.

Factor 2: the shape

Shapes spread weight differently. Our engine gives these face-up sizes for a 1 ct diamond at typical proportions:

• Round: 6.5 mm
• Oval: 8.0 x 5.7 mm
• Pear: 8.7 x 5.6 mm
• Marquise: 10.5 x 5.2 mm
• Emerald cut: 7.1 x 5.0 mm
• Princess: 5.6 x 5.6 mm

Long shapes cover more finger. Square shapes with sharp corners hold more weight in the corners and in a deeper pavilion, so they face up smaller. The diamond size chart shows every shape at every weight.

Factor 3: depth

A stone's pavilion, the cone underneath, holds much of its weight. Cut it deeper and the same weight produces a smaller top. For a 1 ct round diamond, our engine gives 6.6 mm at 58% depth and 6.3 mm at 65% depth.

Run it the other way with a measured stone. Two diamonds both 6.5 mm across, one 3.8 mm deep and one 4.2 mm deep, estimate at 0.98 ct and 1.08 ct. Same face-up size, about 0.10 ct difference in weight. If you are paying by the carat, the shallower one gives you the same look for less weight.

Factor 4: the girdle and other hidden weight

The girdle is the thin edge around the widest part of a stone. A thick girdle adds weight but no visible size. So do a bulging pavilion and a high crown. On a 6.4 mm round diamond, 3.9 mm deep, our estimate moves from 0.97 ct to 1.03 ct when we tick "thick girdle" in the calculator.

Colored stones are especially prone to hidden weight. Many are cut to save as much of the rough crystal as possible, with deep, rounded pavilions. A colored stone that "faces up small" for its weight is very common, and is not a defect in itself, but it is a reason to ask for measurements.

Why CZ catches buyers out

CZ is about 65% denser than diamond. A CZ the size of a 1 ct diamond really weighs about 1.65 ct. That creates two kinds of confusing listings. Some state the real CZ weight, which sounds big for the size. Others state a "diamond equivalent" figure, which matches the size but not the weight. Neither is wrong on its own, but you need to know which you are reading. Comparing a "2 ct" CZ ring with a "2 ct" diamond ring without checking can mislead you either way.

Reading a listing carefully

1. Is the weight real or equivalent? Look for DEW, "diamond equivalent", "ct DEW" or "looks like". If you see any of them, the figure describes size.
2. Is it one stone or the total? TW, TCW and cttw mean total weight across all stones.
3. Is it weighed or estimated? "Approx." and "est." usually mean the stone is set and the weight came from measurements.
4. Are there measurements? Millimetres are the most useful number for judging size. If a listing gives weight but no measurements, ask for them.
5. Is the description specific? "Simulated diamond" and "lab diamond" are different things. Ask what the material is if the wording is vague.

None of this is about doubting sellers. Most listings are written in good faith with trade conventions that buyers do not always know. Knowing the conventions lets you compare fairly.