Specific Gravity Measurement
Hydrostatic weighing and heavy-liquid methods for measuring specific gravity – the density-based property used to identify and confirm gem species.
Introduction
Specific gravity (SG) – also called relative density – is the ratio of a substance's weight
to the weight of an equal volume of water. It is a fundamental physical property that helps
identify gem species, particularly when refractive index and optical tests are inconclusive.
SG measurement is a Foundation-required skill (Gem-A FS5) and a standard part of any
complete gemstone identification sequence.
Hydrostatic Balance Principle
The hydrostatic method applies Archimedes' principle: a body immersed in a liquid experiences
an upward buoyant force equal to the weight of the displaced liquid.
Formula:
SG = W_air / (W_air − W_water)
Where:
- W_air = weight of the stone in air (grams)
- W_water = apparent weight of the stone when fully immersed in water (grams)
- (W_air − W_water) = buoyancy force = weight of displaced water = weight of equal
volume of water
The denominator is the key: it gives the weight of a volume of water equal to the stone's
volume, and dividing the stone's mass by that gives the density ratio relative to water.
Farrimond (1994) provides a practical account of this method for gemmological use:
Journal of Gemmology 24(3), 161–163 (DOI: 10.15506/jog.1994.24.3.161) [VERIFIED].
Equipment Setup
A standard hydrostatic SG setup uses readily available laboratory equipment.
Required Components
- Analytical balance – single-pan electronic or two-pan beam balance; ±0.01 g
resolution minimum (±0.001 g preferred for stones below 0.5 g) - Bridge or cradle frame – mounted over or beside the balance pan; holds the beaker
of water while the stone hangs suspended below on a wire - Fine wire sling or fibre-basket – suspends the stone fully immersed without
touching the beaker walls or base - Beaker of distilled water with one drop of wetting agent (household detergent) to
reduce surface tension and prevent air bubbles - Thermometer – for temperature corrections if precision is required
- Tweezers – for stone handling; avoid fingers which add grease
Procedure
- Tare the balance with the wire sling hanging freely in air above the beaker.
- Place the stone in the sling, lower it so it hangs freely in air. Record W_air.
- Raise the beaker of distilled water so the stone is fully immersed – check no
air bubbles are visible on the stone surface; gently agitate if needed. - Record W_water (the apparent weight under water – numerically less than W_air).
- Calculate: SG = W_air / (W_air − W_water).
- Repeat at least twice; take the mean.
Worked example: W_air = 2.50 g; W_water = 1.60 g
→ SG = 2.50 / (2.50 − 1.60) = 2.50 / 0.90 = 2.78 → consistent with beryl.
Temperature Corrections
Water density varies with temperature. At 4 °C the density is 1.0000 g/cm³ (the
standard reference). At 25 °C it is 0.9971 g/cm³.
For most gemmological work (accuracy to ±0.02 SG units) no correction is needed at
typical room temperature (18–25 °C). For precision work, multiply the result by the
actual water density at the measured temperature.
Heavy Liquids for Rapid SG Bracketing
Heavy liquids are dense organic or inorganic liquids used for density-based separation
without weighing. A stone denser than the liquid sinks; a stone less dense floats; a stone
of equal density suspends (hovers mid-liquid).
This gives a rapid qualitative SG bracket in seconds. Liquids may be diluted (with acetone
for organic liquids, or water for sodium polytungstate) to adjust density to intermediate
values. Shannon (1985) described the gemmological use of heavy liquids in Gems & Gemology
(DOI: 10.1080/00357529.1985.11764366) [PARTIALLY_SUPPORTED – title and DOI confirmed;
abstract not retrieved].
| Liquid | Common Name | Density (g/cm³) | Hazard Class | Status |
|---|---|---|---|---|
| Di-iodomethane (CH₂I₂) | Methylene iodide | ~3.32 at 20 °C | Toxic vapour; photodegrades; stains skin | Still in common use – handle in fume cupboard |
| Bromoform (CHBr₃) | Bromoform | ~2.89 at 20 °C | Toxic; suspected carcinogen; volatile | Being phased out in many laboratories |
| Clerici solution (thallium formate–malonate) | Clerici | Up to ~4.2 (adjustable) | Acutely toxic – thallium absorbed through skin | Banned or restricted in many jurisdictions; avoid |
| Sodium polytungstate (Na₆[H₂W₁₂O₄₀]) | SPT | ~2.7–3.1 (adjustable with water) | Non-toxic; water-soluble | Modern alternative; does not reach 3.32+ SG range |
Safety
SG Reference Values
These SG values are used to evaluate hydrostatic results and to predict sink/float behaviour
in heavy liquids. Values from the Gem-A Foundation constants table.
| Species | SG Range | Behaviour in Di-iodomethane (SG 3.32) | Behaviour in Bromoform (SG 2.89) | Notes |
|---|---|---|---|---|
| Diamond | 3.52 | Sinks | Sinks | Very consistent; cubic |
| Corundum (ruby/sapphire) | 3.80–4.05 | Sinks | Sinks | Wide range due to Fe/Ti content |
| Spinel (natural) | 3.58–3.61 | Sinks | Sinks | Isotropic |
| Spinel (Verneuil synthetic) | 3.61–3.67 | Sinks | Sinks | Slightly higher than natural |
| Topaz | 3.50–3.60 | Sinks | Sinks | Perfect basal cleavage – handle carefully |
| Peridot | 3.32–3.37 | Floats / suspends | Sinks | SG overlaps di-iodomethane closely |
| Jadeite | 3.30–3.36 | Floats / suspends | Sinks | Aggregate material |
| Nephrite | 2.80–3.10 | Floats | Floats / suspends | Lower than jadeite – useful diagnostic |
| Beryl (all varieties) | 2.65–2.80 | Floats | Floats | Includes emerald, aquamarine, morganite |
| Quartz (crystalline) | 2.65 | Floats | Floats | Very consistent; useful balance calibration standard |
| Amber | 1.05–1.10 | Floats | Floats | Floats in saturated salt water (SG ~1.13) |
| Glass (paste) | 2.0–4.2 | Variable | Variable | Wide range depending on lead content |
Sources of Error
The following errors are common in hydrostatic SG measurement:
Measurement Errors
- Air bubbles on the stone surface – reduce the apparent weight of displaced water,
giving a falsely high SG reading. Remove by gently agitating or by using a wetting
agent in the water. - Mounted stones – metal settings introduce unknown mass; the method is unreliable
for set stones without dismounting. - Porous or treated stones – water absorption (opal, turquoise, some coral) alters
the reading over time as the stone absorbs liquid. - Very small stones – weighing error becomes proportionally large; minimum useful
stone weight is approximately 0.15–0.20 g for ±0.01 g balance resolution. - Fracture-filled stones – filler material (glass, resin) changes the apparent SG
from the host mineral.
Balance Calibration
- Check balance calibration before each session using a stone of known SG.
- Quartz (SG 2.65) is a convenient and inexpensive reference standard.
- If readings on a reference stone deviate by more than ±0.02, recalibrate the balance
before continuing.
Sources
Key citations for this topic:
- Farrimond, T. (1994). "Hydrostatic measurement of specific gravity." The Journal of
Gemmology 24(3), 161–163. DOI: 10.15506/jog.1994.24.3.161 [VERIFIED] - Walton (1951). "A Specific Gravity Balance." The Journal of Gemmology 3(2), 43.
DOI: 10.15506/jog.1951.3.2.43 [VERIFIED] - Mitchell (1980). "Anderson on Heavy Liquids." The Journal of Gemmology 17(4), 230.
DOI: 10.15506/jog.1980.17.4.230 [VERIFIED] - Shannon (1985). "Determining Specific Gravity Using Heavy Liquids." Gems & Gemology.
DOI: 10.1080/00357529.1985.11764366 [PARTIALLY_SUPPORTED – title and DOI confirmed;
abstract not retrieved] - Munsterman, D. & Kerstholt, S. (1996). "Sodium polytungstate, a new non-toxic alternative
to bromoform in heavy liquid separation." Review of Palaeobotany and Palynology 91,
417–422. DOI: 10.1016/0034-6667(95)00093-3 [VERIFIED] - Read, P. G. (ed.). Gems 7th ed., chapter "Specific gravity, density and relative
density." DOI: 10.4324/9780080507224-12 [VERIFIED]