Specific Gravity Testing at Home: The Hydrostatic Weighing Method
Specific gravity is one of the more underused identification tests in a beginner rockhound’s toolkit, mostly because it sounds like it needs lab equipment. It doesn’t. A kitchen scale, a cup of water, and a bit of thread are enough to get a genuinely useful reading.
What specific gravity actually is
Specific gravity (SG) is the ratio between a specimen’s weight and the weight of an equal volume of water. Because water’s density is very close to 1 gram per cubic centimeter, SG works out to a number that’s essentially the specimen’s density in g/cm³, without you needing to measure volume directly at all. A specimen with SG 2.65 (quartz) is 2.65 times as heavy as the same volume of water; a specimen with SG 7.5 (galena) is nearly three times denser again.
Why it's a useful test
Two minerals can look almost identical — similar color, similar hardness, similar luster — and still have very different densities, because density depends on which atoms make up the mineral and how tightly they’re packed in the crystal lattice. A metallic-looking gray mineral with SG around 5 is far more likely to be galena or pyrite than a silicate; ordinary rock-forming minerals like quartz, feldspar, and calcite almost all cluster in a narrower 2.5–3.0 range. SG narrows the field fast, especially combined with hardness and streak.
The hydrostatic method, step by step
The classic approach uses Archimedes’ principle: an object submerged in water experiences a buoyant force equal to the weight of the water it displaces, so the difference between a specimen’s weight in air and its (lower) apparent weight underwater equals the weight of the water it pushed aside.
- Weigh the dry specimen in air. Use the most precise scale you have — a digital scale reading to 0.01g is far better than a kitchen scale reading to whole grams, especially on small specimens.
- Tie a fine thread around the specimen and suspend it so it hangs freely without touching the sides or bottom of a water-filled container.
- Weigh it again while fully submerged. Make sure no air bubbles are clinging to the surface — tap the thread gently or brush the specimen underwater first, since trapped bubbles will throw the reading off.
- Subtract the submerged weight from the air weight to get the displaced-water weight, then divide the air weight by that difference. That's your specific gravity.
Our specific gravity calculator does that division for you and sorts the result into a density band with example minerals, so you don’t need to keep a reference table on hand.
Sources of error
Porous specimens are the biggest issue — if a specimen absorbs water during the test, its apparent weight in water changes over time and the result drifts. Weigh quickly and note if the specimen feels notably lighter or heavier the longer it sits submerged. A scale that isn’t properly zeroed, a thread thick enough to meaningfully affect the reading, or a specimen touching the container walls will all introduce smaller errors. On very small specimens, even a fraction of a gram of error can shift the calculated SG noticeably, so this method works best on pieces at least a few grams in weight.
What to do with the result
Treat a specific gravity reading as one data point, not a final answer. A result of 2.65 is consistent with quartz, but so are a handful of other minerals in that density range — pair it with hardness, streak, luster, and cleavage before settling on an identification, and remember that heavily included or partially altered specimens can read a bit off from a textbook value for a pure sample.