Specific Gravity of Gemstones

Specific gravity (SG) compares a gem's density with the density of water. A stone with an SG of 4.00 weighs four times as much as the same volume of water. Every gem species falls within a narrow SG range, so the number helps separate stones that look alike, and it can be measured without harming the gem.

Density relative to water

Density is mass per unit of volume. Specific gravity is the same idea expressed as a ratio: the weight of the gem divided by the weight of an equal volume of water at about 4 degrees Celsius, where water is densest. Because it is a ratio, SG has no units. In grams per cubic centimeter, density and SG come out to the same number, so a sapphire with an SG of 4.00 has a density of 4.00 g/cm3.

SG depends on two things: the atoms that make up the mineral and how tightly they pack. Heavy elements raise SG. Zircon, which contains zirconium, has an SG near 4.7. Tightly packed structures also raise it, which is why diamond (pure carbon) has an SG of 3.52 while graphite, also pure carbon in a looser sheet structure, sits near 2.2. The diamond chemistry page explains that difference.

Why SG matters to buyers

Two stones of the same carat weight can look quite different in size. A one-carat cubic zirconia (SG about 5.6 to 6.0) is noticeably smaller than a one-carat diamond (3.52), because the CZ packs more weight into less space. A one-carat amethyst (2.65) looks larger than a one-carat sapphire (4.00). When comparing gems by carat, keep SG in mind: low-SG stones give more size per carat.

Hydrostatic weighing

The standard way to measure SG is hydrostatic weighing, based on Archimedes' principle. A gem submerged in water is pushed up by a force equal to the weight of the water it displaces. Weigh the gem in air, then weigh it suspended in water. The loss in weight equals the weight of an equal volume of water.

The calculation is simple:

1. Weigh the gem in air.
2. Weigh the gem fully submerged in water, hanging in a wire basket that does not touch the container.
3. Subtract the weight in water from the weight in air.
4. Divide the weight in air by that difference.

Take a gem that weighs 2.00 ct in air and 1.50 ct in water. The difference is 0.50 ct, and 2.00 divided by 0.50 gives an SG of 4.00, consistent with corundum.

A precise carat scale with a hydrostatic attachment works well for loose stones above about one carat. Small stones give less reliable results because tiny weighing errors make a large difference. Gemologists often add a drop of detergent to the water to break surface tension and remove air bubbles clinging to the stone. Mounted gems cannot be measured this way, since the metal throws off the reading.

Heavy liquids

Heavy liquids give a faster, rougher answer. These are dense fluids prepared to known SG values. Drop a gem into a liquid: if it sinks, its SG is higher than the liquid; if it floats, lower; if it hangs in the middle, it matches.

Traditional gemological sets included liquids around 2.57, 2.62, 2.67, 3.05 and 3.32. Diiodomethane (methylene iodide) has an SG of about 3.32 and can be diluted to lower values. Bromoform, near 2.89, was also widely used. Clerici solution can reach about 4.2 but is highly toxic. All of these liquids are hazardous, and many labs have moved toward less toxic sodium polytungstate solutions or use hydrostatic weighing instead.

Heavy liquids are good for sorting parcels and for testing small stones. Porous gems such as opal and turquoise should not go in them, since they can absorb the liquid. Treated stones with fillers or oils should also stay out.

SG chart of common gems

Exact ranges vary a little between references. Mineral groups such as garnet and tourmaline cover wider ranges because their chemistry varies.

Using SG to separate look-alikes

SG earns its place in identification when two gems look the same and have close refractive indices. A few pairs where it helps:

• Diamond (3.52) and its simulants. Cubic zirconia (5.6 to 6.0) is much heavier, while synthetic moissanite (about 3.22) is slightly lighter. In 3.32 liquid, moissanite floats and diamond sinks.
• Quartz (2.65) and topaz (3.53). Yellow citrine and yellow topaz are easily confused, and their SG values are far apart.
• Amber (about 1.08) and plastic imitations. Amber floats in a strong salt solution (SG around 1.13), while many plastics and all glass sink. The amber page covers other amber tests.
• Jadeite (about 3.34) and nephrite (about 2.95). The two minerals sold as jade separate readily in 3.05 liquid.
• Spinel (3.60) and corundum (4.00). Red spinel and ruby share a color, but corundum is noticeably heavier.

Glass imitations can be made with SG values anywhere from about 2.3 to over 4, so SG alone does not rule them out. It works best combined with refractive index, optic character and inclusions. The guide on how to identify gemstones shows how the tests fit together, and the gemological instruments page describes the scales and accessories.

Frequently asked questions

Is specific gravity the same as density?

Nearly. Specific gravity is density divided by the density of water, so it has no units. In grams per cubic centimeter, the two numbers match.

Why does a one-carat cubic zirconia look smaller than a one-carat diamond?

Cubic zirconia has an SG of about 5.6 to 6.0, compared with 3.52 for diamond. The same weight fits into a smaller volume, so the CZ measures smaller across.

Can specific gravity be measured on a gem set in jewelry?

Not accurately. The metal changes the weight and volume, so hydrostatic weighing and heavy liquids only work on loose stones.

Which gem has the lowest specific gravity?

Amber, at about 1.05 to 1.10, is among the lightest gem materials. It barely sinks in fresh water and floats in salty water.