Specific Gravity Calculator
Archimedes' test, on a kitchen scale. It leaves no mark, which is why it works on a polished stone.
Weigh the specimen dry, then weigh it hanging fully submerged in water, and divide the dry weight by the difference. That ratio is the specific gravity: amber is 1.05, opal 2.1, quartz 2.65, jadeite 3.3, pyrite 5.0, hematite 5.3 and gold 19.3. It is non-destructive, works on polished stones, and needs no equipment beyond a scale that reads to a tenth of a gram.
2.64
The ordinary range: quartz, feldspar, calcite, turquoise.
12.4 ÷ (12.4 − 7.7) = 2.64
- Quartz2.65
No cleavage at all, curved conchoidal fracture, inert to acid.
Mohs 7 · streak white or none
- Amethyst2.65
Quartz, so hardness 7 — which is what separates it from purple fluorite.
Mohs 7 · streak white or none
- Agate / chalcedony2.58–2.64
Waxy rather than glassy lustre, with concentric banding and no acid reaction.
Mohs 6.5–7 · streak white
- Turquoise2.6–2.8
Resists a knife, where dyed howlite gives way to white powder.
Mohs 5–6 · streak white to pale green
- Labradorite2.68–2.72
Two cleavage directions near 90°, and a flash that dies when you tilt it.
Mohs 6–6.5 · streak white
- Calcite2.71
Effervesces briskly in vinegar; a clear piece doubles a printed line.
Mohs 3 · streak white
- Howlite2.53–2.59
White beneath any dye. The usual stand-in for turquoise.
Mohs 3.5 · streak white
- Lapis lazuli2.7–2.9
Metallic pyrite flecks embedded in the blue, with white calcite that fizzes.
Mohs 5–6 · streak pale blue
- Obsidian2.3–2.6
Amorphous glass — no cleavage, and translucent brown at a thin edge.
Mohs 5–5.5 · streak white
Several candidates share this density. A hardness bracket will separate them faster than anything else.
Density, measured with a kitchen scale and a cup of water
Specific gravity is the most underused test available to anyone identifying a stone, and it is the one that works when hardness cannot be run — on a polished cabochon, a carving, a bead, anything where a scratch would show. It is also the test that separates jadeite from nephrite from serpentine, which nothing else does reliably at home.
The method is Archimedes' and it produces a ratio, which is why the units do not matter. Grams, ounces, anything: as long as both readings use the same scale, the answer comes out the same.
The method, step by step
The only fiddly part is the submerged weighing, and a length of fine thread solves it. What matters is that the specimen hangs completely under the water and touches nothing — not the sides, not the bottom — because any contact transfers weight to the container and ruins the reading.
- Clean and dry the specimen, then weigh it. Record the figure.
- Half fill a cup with water and stand it on the scale. Tare to zero.
- Hang the specimen from a fine thread so it is fully submerged and touching nothing.
- Read the figure the scale now shows — that is the weight of water displaced.
- Specific gravity is the dry weight divided by that displacement figure.
Where the errors come from
Three things spoil a reading, and all three are avoidable. Air bubbles clinging to the specimen make it read light, so wet it first and flick the bubbles off. Contact with the container transfers weight and makes it read heavy. And a porous stone drinks water, which makes it read light — turquoise, unstabilised material and hydrophane opal are all genuinely absorbent, and on those the measurement is unreliable rather than merely imprecise.
Scale resolution matters too. On a specimen under a few grams, a scale reading to whole grams cannot produce a useful ratio; a tenth of a gram is the practical minimum, and a hundredth is better. This is why the tool matches against a tolerance rather than looking for an exact hit — pretending to more precision than a kitchen scale delivers would return a confidently empty list.
The separations this test makes that nothing else does
Jade is the headline case. Jadeite at 3.30–3.38, nephrite at 2.90–3.03 and serpentine at 2.44–2.62 look similar, polish similarly and cannot be told apart by eye — and they differ in value by orders of magnitude. Density separates all three cleanly in five minutes.
Amber is the second. At 1.05–1.09 it is lighter than water is dense, which is why the saturated salt water float test works at all, and why plastic and glass imitations sink. Opal is the third: at about 2.09 it is remarkably light for a gem material, and a heavy stone sold as opal is not one.
And at the far end, gold's 19.3 against pyrite's 5.0 is the largest useful gap in the field. Anyone who has weighed both once never confuses them again.
| Material | Specific gravity | What it separates it from |
|---|---|---|
| Amber | 1.05 – 1.09 | Plastic and glass, which sink in brine |
| Opal | 2.05 – 2.15 | Almost every other gem material |
| Selenite (gypsum) | 2.3 | Calcite at 2.71 |
| Obsidian | 2.3 – 2.6 | Jet at about 1.3 |
| Serpentine | 2.44 – 2.62 | Nephrite jade at 2.90 and above |
| Turquoise | 2.6 – 2.8 | Resin composites, far lighter |
| Quartz | 2.65 | The baseline for 'ordinary' |
| Calcite | 2.71 | Quartz, marginally |
| Labradorite | 2.68 – 2.72 | Glass imitations |
| Nephrite jade | 2.90 – 3.03 | Serpentine below, jadeite above |
| Fluorite | 3.0 – 3.25 | Quartz and amethyst at 2.65 |
| Jadeite jade | 3.30 – 3.38 | Nephrite and every substitute |
| Topaz | 3.5 – 3.6 | Quartz |
| Malachite | 3.6 – 4.0 | Pigmented resin at about 1.2 |
| Garnet | 3.9 – 4.2 | Most silicates |
| Corundum | 3.95 – 4.1 | Glass |
| Pyrite | 4.9 – 5.2 | Gold, by a factor of nearly four |
| Hematite | 5.2 – 5.3 | Manufactured ferrite, and magnetite by streak |
| Gold | 19.3 | Everything |
Ranges rather than points, because trace substitutions within a mineral shift the figure slightly — and because a kitchen scale carries real error on a small specimen.
Questions we are asked
- How do I measure specific gravity at home?
- Weigh the specimen dry. Then stand a cup of water on the scale, tare to zero, and lower the specimen in on a thread so it hangs fully submerged and touches nothing — the figure the scale shows is the water displaced. Divide the dry weight by that figure. The result is a ratio, so the units do not matter.
- Why is my specific gravity reading too low?
- Usually air bubbles or porosity. Bubbles clinging to the specimen make it displace more water than it should, and a porous stone such as unstabilised turquoise or hydrophane opal absorbs water outright. Wet the specimen and flick the bubbles off before reading; on genuinely porous material, the test is unreliable.
- Can I use specific gravity on a polished or set stone?
- On a polished stone, yes — it is entirely non-destructive, which is its main advantage over a hardness test. On a set stone, no: the metal and any other stones are part of the weight, so the figure describes the whole piece rather than the gem.
- How can I tell jadeite from nephrite?
- Density is the practical answer at home. Jadeite is 3.30–3.38, nephrite 2.90–3.03 and serpentine 2.44–2.62 — three materials that look similar, polish similarly and differ enormously in value. A kitchen scale and a cup of water separate them in about five minutes.
- What scale do I need?
- One that reads to a tenth of a gram at minimum, and a hundredth is better. On a specimen of a few grams, a scale that reads only whole grams cannot produce a ratio precise enough to distinguish anything useful.
Before You WeighRead the stone itself
Density separates the candidates hardness could not. Photograph the specimen and the instrument gives you the shortlist to test in the first place.