Why weighing something in water tells you its density
Archimedes’ principle says an object under water is pushed upward by a force equal to the weight of the water it displaces. Hang a ring from a scale and lower it into a cup of water, and the scale reads less than it did in air. The difference is not a property of the ring’s surface or shape — it is exactly the weight of a ring-shaped volume of water.
That turns two readings into a density. A piece that weighs 10.00 g in air and 9.45 g in water lost 0.55 g, so it displaced 0.55 g of water, which is about 0.55 cm³. Ten grams in 0.55 cm³ is a specific gravity of 18.2. The arithmetic is exact: weight in air divided by the loss in water. Nothing has been estimated, and no conversion factor is hiding in it. Specific gravity is density relative to water, and since water at room temperature is about 0.998 g/cm³, the two numbers agree to within a fraction of a percent — far finer than any home scale can see.
How to take the two readings
Weigh the piece dry first. Then put a cup of water on the scale, zero it, and lower the piece on a thin thread until it hangs fully submerged, touching neither the sides nor the bottom. The scale now shows the weight of the displaced water directly; subtract that from the air weight and you have the in-water reading this calculator asks for. Alternatively, suspend the piece from a hook under the scale. Either way, knock off any air bubbles clinging to it — a bubble is a tiny float and makes the piece look less dense than it is.
Use as fine a scale as you can. The calculator shows a range rather than a single figure because the result depends on a small difference between two readings, and a small difference magnifies error. A 2 g gold ring loses only about 0.1 g in water. On a scale that reads to 0.01 g, half a step of error on each reading can move that 0.1 g by ten percent, which swings the specific gravity by about two units either way. The range is the honest output; the middle figure on its own overstates what the scale can see.
Why there is no figure for 14K gold
Pure gold has a density of 19.32 g/cm³, and every reference agrees. Karat gold does not have one density. A 14K alloy is 58.5% gold by weight, and the remaining 41.5% can be silver, copper, zinc, nickel or palladium in proportions each maker chooses. Silver is about 10.5 g/cm³ and copper about 9.0, so two 14K rings with different alloys have measurably different densities. Tables that give one “14K density” are quoting one alloy and presenting it as all of them.
This page therefore places a reading among materials that do have a single settled value — pure metals, and the gem minerals already used on the gemstone weight pages — and stops there. A reading that falls between silver and pure gold is consistent with karat gold, but it is equally consistent with a dozen other things. A specific gravity test is most useful as a rejection test: a piece sold as 18K that reads 8.5 is not 18K, whatever the stamp says.
What gets past it
Any construction that changes the volume without changing the metal fools it. Hollow chains and tubes read low. A stone, a steel clasp spring or a glued-in pearl changes the reading by an amount that depends on its size. Plating is invisible to it: the plate is so thin that a gold-plated brass bangle reads as brass. Worst of all, tungsten is within half a percent of gold’s density, which is precisely why tungsten has been used inside counterfeit bars. A matching specific gravity is consistent with genuine metal; it is not proof of it, and the only proof is an assay.