Specific Gravity Testing — How to Weigh Gems in Water and Identify Any Stone
By CeylonGemLuxury Editorial Team · 8 min read
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Specific gravity (SG) is the ratio of a gem's weight to the weight of an equal volume of water. It is a pure physical constant — you cannot fake it, and you cannot change it through heat treatment, filling, or dyeing. Every mineral species has a characteristic SG range, just as it has a characteristic RI. Used together, RI and SG are the two most powerful identification measurements available to the gem buyer without a full laboratory.
The beauty of SG testing is that it works on rough gems, stones without flat facets, and even gems that are too small for a reliable refractometer reading. If a gem can be weighed (which requires only a $30 digital scale), it can be specific-gravity tested.
SG Reference Table for Ceylon Gem Species
| Gem Species | SG Range | "Heavy" Feel? |
|---|---|---|
| Zircon (blue Ceylon) | 4.65–4.70 | Yes — distinctly heavy |
| Ruby / Blue Sapphire (corundum) | 3.99–4.01 | Noticeably heavy |
| Yellow Sapphire | 3.99–4.01 | Same as blue sapphire |
| Chrysoberyl / Alexandrite | 3.71–3.75 | Moderately heavy |
| Spinel | 3.58–3.61 | Moderate |
| Tourmaline | 3.02–3.26 | Moderate (variable) |
| Moonstone (feldspar) | 2.55–2.57 | Light |
| Quartz (all varieties) | 2.65–2.66 | Light |
| Glass (simulant) | 2.30–4.50 | Variable — wide range |
How to Measure Specific Gravity: Step by Step
The hydrostatic (Archimedes) method uses Archimedes' principle: a body immersed in liquid loses weight equal to the weight of liquid it displaces. The formula is:
- Weigh the gem in air on your digital scale. Record this as Wair. Note: the gem must be dry.
- Set up the hydrostatic suspension. Place a beaker of water on the scale pan. Build a wire bridge across the beaker so you can suspend the gem from the scale hook into the water without the gem touching the beaker.
- Tare (zero) the scale with the beaker of water in place.
- Suspend the gem in the water using a fine wire loop or thin nylon thread attached to the scale hook. The gem must be fully submerged and not touching the beaker walls or bottom.
- Read the weight displayed. This is Wwater (the apparent weight in water, which will be less than in air).
- Calculate: SG = Wair ÷ (Wair − Wwater). For example: a sapphire weighing 2.000g in air and 1.500g in water → SG = 2.000 ÷ (2.000 − 1.500) = 2.000 ÷ 0.500 = 4.00. This confirms corundum (sapphire / ruby).
Accuracy Tips
Small gems (under 1 carat / 0.2g) require a scale accurate to 0.001g for usable SG results. A 0.01g scale introduces too much rounding error. The test works best on stones above 0.5 carats. Ensure no air bubbles cling to the gem during the water weighing — a tiny air bubble creates a buoyancy error. Tap the gem gently after submerging to dislodge any bubbles.
Use distilled or filtered water at room temperature (20°C) for the most accurate results. The SG of water varies slightly with temperature — at 25°C it is 0.997 rather than 1.000, which is close enough for gem identification purposes but worth knowing.
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