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What does a coloured stone weigh?

The same size stone weighs different carats in different minerals. A ruby is denser than a diamond, so it looks smaller for its weight.

Corundum — denser than diamond, so it looks smaller per carat.

mm
mm

Through the stone, top to bottom. If it is set and you cannot measure it, the estimate will be rough.

mm

Estimated weight, ruby

0.84 ct

Estimated from the outline and the mineral's density. Real stones vary with how they are cut, and a set stone whose depth cannot be measured varies more — this is an approximation, not a scale reading.

Weight

0.84 ct

0.168 g

Specific gravity

4.00

water = 1

Same size in diamond

0.74 ct

for comparison

Density vs diamond

1.14×

A oval ruby measuring 7 × 5 × 3.4 mm weighs roughly 0.84 ct. The identical stone cut in diamond would weigh 0.74 ct — less, because ruby is denser than diamond.

One carat, in each stone

Every stone below weighs exactly one carat, drawn to the same scale as a oval. The carat is a unit of weight, so a denser mineral spends it on fewer millimetres.

Opal

6.7 mm

Emerald

6.2 mm

Diamond

5.7 mm

Ruby

5.5 mm

Cubic zirconia

4.9 mm

This is why coloured stones are bought by millimetre rather than by carat. Asking for a one-carat sapphire to match a one-carat diamond gives you a visibly smaller stone; asking for a 5.7 mm sapphire gives you one that matches, and it will weigh about 1.14 ct.

Reference: a one-carat Ruby of this shape measures about 5.5 mm wide.

Why a one carat ruby is smaller than a one carat diamond

Because a carat is a unit of weight, the same weight in a denser mineral occupies less space. Ruby and sapphire are corundum, with a specific gravity around 4.0 against diamond’s 3.52 — roughly 14% denser. So a one carat ruby is visibly smaller than a one carat diamond, and matching them by carat weight guarantees a mismatch.

It runs the other way too. Emerald and aquamarine sit around 2.7, so they look larger per carat than diamond. Opal at about 2.15 is the lightest common gem, and cubic zirconia at 5.7 is much the heaviest — a CZ cut to the same dimensions as a one carat diamond weighs closer to 1.6 carats, which is one way the two get told apart.

Buy coloured stones by millimetre

This is why the coloured stone trade quotes millimetres rather than carats. If you are matching a sapphire to a diamond in the same setting, ask for the millimetre size, not the weight. A 6.5 mm round sapphire will sit correctly beside a one carat round diamond and weigh about 1.14 carats.

Weight estimated from measurements uses the standard trade factors and assumes a normally cut stone. It is good to a few percent for a loose stone you can measure properly. A set stone whose depth cannot be reached will be rougher, because depth is the dimension the estimate is most sensitive to.

Common questions

Why does a 1 carat sapphire look smaller than a 1 carat diamond?

Sapphire is corundum, with a specific gravity around 4.0 against diamond’s 3.52, so it is about 14% denser. The same weight therefore takes up less room. To match sizes, compare millimetres rather than carats.

How do you calculate the carat weight of a gemstone?

Multiply length by width by depth in millimetres, then by a factor for the shape — 0.0061 for a round, 0.0062 for an oval, 0.0083 for a princess — which gives the weight as if it were diamond. Then scale by the mineral’s specific gravity divided by 3.52.

Is moissanite heavier or lighter than diamond?

Lighter. Moissanite has a specific gravity of about 3.21 against diamond’s 3.52, so a moissanite cut to the same size weighs roughly 9% less. This is why moissanite is sold by millimetre size rather than by carat weight.

Next

What is my gold worth?

Weigh it, pick the karat, and see the value of the metal at the live gold price — then what a buyer is likely to hand over, which is a different number.

What is my silver worth?

Sterling, coin, Britannia and fine silver at the live silver price — including why small silver items are often worth less than the trouble of selling them.

What is platinum or palladium worth?

PT950, PT900 and PD950 at live prices. Both metals are denser than gold, which is why the weight surprises people.