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The Specific Gravity Kit, Explained
Density: the reading look-alikes cannot fake
Two stones can share a refractive index and still not be the same mineral. Density is the property that refuses to cooperate with disguises: the same volume of corundum and the same volume of glass simply do not weigh the same. The specific gravity kit exists to read that difference on a bench.
Hydrostatic weighing: the same stone on the scale, dry and in water.
Hydrostatic weighing, the bench method
You weigh the stone dry. Then you weigh it again suspended in water, touching nothing. The water pushes the stone up with a force equal to the weight of the water it displaces, so the two readings together give the stone's density:
| Step | Reading |
|---|---|
| Weigh the stone in air | Wdry |
| Weigh it suspended in water | Wwater |
| Specific gravity | Wdry ÷ (Wdry − Wwater) |
The trade alternative is heavy liquids: sets of fluids at known densities, where a stone sinks, floats or hangs suspended. Fewer benches carry them now; the liability and handling of the liquids outweigh the convenience for most working jewellers.
Why it pairs with the refractometer
Refractive-index twins are common on purpose: glass and assembled stones are formulated to hit fashionable numbers. When the refractometer says two candidates match, density usually does not, which is why the professional sequence runs both numbers before believing either. "Look-alikes with identical RI: density never lies" is not a slogan. It is the working reason the kit stays on the bench.
What it cannot do
Mounted stones resist the method: a setting traps air and adds metal to the maths. Filled or porous material shifts the number by shifting what is actually being weighed. And on tiny stones the scale error grows relative to the stone, so small melee is better read by other instruments first.
Learn it hands-on
- GT200 Gem Identification Methods: hydrostatic weighing as a bench skill, on the full stone set.
- The refractometer, explained: the number this kit double-checks.
- All eight identification tools: what each one measures and who realistically needs it.
- Gem materials index: densities for 83 materials, the list the readings get checked against.
Learn this properly, gemmology at JDMIS, Singapore
Everything on this page is taught hands-on at JDMIS with real stones:
- Gemmology: Secrets of the Gem & Jewellery Trade (GT100): hydrostatic weighing is a core GT100 bench skill; density never lies
- Gem Identification Methods (GT200): hands-on instrument work: refractometer, specific gravity, microscope and UV, the way professionals actually identify stones.
Free tools while you read: Gem Globe (where stones come from) · Name That Gem (test your eye) · our guide to every Singapore gem laboratory.
Educational content by the gemmology faculty of the Jewellery Design & Management International School (JDMIS), Singapore. Gemmological constants quoted are standard reference values.