Specific gravity (SG) is the ratio of a material's density to the density of water. A stone with an SG of 4.00 is four times as dense as water. Like refractive index, it is a property of the material rather than the cut, it is fixed for each gem species, and it can be measured without touching the stone with anything harder than water.
It is the laboratory's answer to a question the refractometer cannot handle: what is this bead, this cabochon, this carved piece with no flat facet?
How it is measured
The stone is weighed in air on a precision balance. It is then suspended in water — on a fine wire or in a small basket hanging from the balance — and weighed again. Immersed, it weighs less by the weight of the water it displaces. Specific gravity is the weight in air divided by the difference:
SG = weight in air ÷ (weight in air − weight in water)
A ruby weighing 2.000 ct in air and 1.500 ct in water has an SG of 2.000 ÷ 0.500 = 4.00. The method is called hydrostatic weighing and dates to Archimedes.
Precision depends on the balance and the stone's size. Large stones give clean results; very small stones (under about 0.5 ct) are hard to measure accurately because a single trapped bubble or a drop of water on the wire skews the figure.
Typical values
| Material | SG |
|---|---|
| Amber | 1.05–1.10 |
| Plastic | 1.05–1.40 |
| Opal | 1.98–2.20 |
| Quartz (amethyst, citrine, rock crystal, agate) | 2.65 |
| Emerald / beryl | 2.67–2.78 |
| Pearl (nacreous) | 2.60–2.85 |
| Coral | 2.60–2.70 |
| Jadeite | 3.30–3.36 |
| Nephrite | 2.90–3.02 |
| Diamond | 3.52 |
| Topaz | 3.53 |
| Chrysoberyl | 3.71–3.75 |
| Garnet (varies) | 3.60–4.30 |
| Corundum (ruby, sapphire) | 3.99–4.01 |
| Zircon | 4.6–4.7 |
| Cubic zirconia | 5.6–6.0 |
| Glass (soda-lime to lead) | 2.3–4.5 |
What SG separates
Quartz from glass. Quartz is 2.65; common glass is about 2.5 and lead glass 3 or more. A "crystal" sphere that reads 3.1 is glass.
Corundum from imitations. Ruby and sapphire are 4.00. Garnet runs lower or higher depending on type, spinel is 3.60, glass varies, synthetic spinel is 3.64. A red stone at 4.00 is corundum (natural or synthetic) or very unusual glass.
Diamond from CZ. 3.52 versus 5.6–6.0. A CZ is more than half again as heavy as a diamond of the same size, which is why CZ "carat weights" mislead.
Jadeite from nephrite and imitations. 3.33 versus 2.95, with glass, serpentine and dyed quartz all lower.
Amber from plastic. Both light, but amber floats in saturated salt water (SG about 1.15) and most plastics sink. Amber vs copal vs plastic.
Beads, malas and carvings
For strung beads the laboratory can measure a single bead if one can be removed, or estimate density of the whole strand from its weight and the calculated volume of its beads. For carvings and spheres the measurement is straightforward. This is why SG is the primary identity test for sphatik malas, coral strands, jade bangles and every other article without a facet for the refractometer.
Limits
SG ranges overlap for some species (several garnets, topaz and diamond), so it rarely stands alone. Porous materials (turquoise, some coral) absorb water and read low. Composite stones, glass-filled stones and stones with large inclusions read as the average of their parts. A treated or filled stone may give an SG slightly off the textbook value, which is a prompt for the microscope rather than a conclusion. How a gemmological laboratory tests a gemstone.
Frequently asked questions
Can SG identify a stone by itself?
Sometimes — a reading of 4.00 on a red stone leaves little doubt. Usually it is combined with the refractometer, polariscope or microscope to reach a conclusion.
Does SG distinguish natural from synthetic?
No. A synthetic ruby has the same SG as a natural one. That is a microscope question.
Can I measure SG at home?
With a balance reading to 0.01 g and a way to suspend the stone in water, yes, for stones large enough to measure. Small stones and beads are difficult; the error from a trapped bubble is large.
Why do some certificates give SG and some not?
Faceted stones are usually identified by RI, which is more precise; SG is reported where it was the deciding test, typically for beads and cabochons. A report should name the tests it used.
Is a heavier stone a better stone?
No. Weight for size is a property of the species, not of quality. A CZ is heavier than a diamond and worth a tiny fraction of it.