Specific Gravity
Also: Relative density, Density ratio, Sp. gravity
Specific gravity (also called relative density) expresses how much heavier a substance is compared with water (density 1.0 g/cm³), and it ranks among the most reliable figures for verifying the authenticity of precious metals.
Specific gravity (the technical term is relative density) is a dimensionless quantity describing the ratio between the density of a substance and the density of pure water at 4 °C (= 1.0 g/cm³). In everyday use the term is often treated as interchangeable with density in g/cm³, because the numerical value is the same. For anyone buying or checking precious metals the figure is indispensable: a genuine gold bar feels noticeably heavier for its size than a fake made from a lighter metal — and specific gravity exposes the deception.
Formula and measurement
Density follows from mass and volume:
ρ = m / V
ρ = density in g/cm³
m = mass in grams (dry weighing)
V = volume in cm³
For irregularly shaped items (coins, broken gold) the volume is hard to measure directly. The most elegant approach is the Archimedean principle: you weigh the piece dry, then again while it is fully submerged in water. The difference equals the weight of the displaced water — from which the volume follows, and from that the density. Our coin authenticity calculator walks through this calculation step by step.
Specific gravities compared
| Metal | Density (g/cm³) | Note |
|---|---|---|
| Osmium | 22.59 | densest stable element |
| Iridium | 22.56 | second densest element |
| Platinum | 21.45 | heavier than gold |
| Gold (999) | 19.32 | the key reference value |
| Tungsten | 19.25 | dangerous counterfeit metal |
| Palladium | 12.02 | markedly lighter than gold |
| Silver (999) | 10.49 | easy to measure |
| Copper | 8.96 | base metal in alloys |
| Aluminium | 2.70 | conspicuously light at once |
Because tungsten (19.25 g/cm³) sits so close to gold's value (19.32 g/cm³), a density test on its own struggles to unmask tungsten forgeries — here extra techniques such as ultrasound or X-ray fluorescence become necessary.
How fineness affects density
Alloys deviate from the pure-metal figures. The lower the fineness, the more strongly the alloying metal pulls the overall density away from pure gold:
- 750 gold (18 carat): roughly 15.5–16.5 g/cm³ (depending on the alloying metal)
- 585 gold (14 carat): roughly 12.9–14.0 g/cm³
- 925 silver (sterling silver): roughly 10.36 g/cm³
Anyone wanting to work out the melt value of scrap gold needs the fineness — and if the fineness is unknown, a combined density-and-weight measurement can help narrow it down. The coin weight checker automatically compares nominal and actual weight for common coins.
Practical authenticity test
- Dry weighing on a precision scale (resolution 0.01 g or better).
- Submerged weighing in distilled water (thread or basket, no air bubbles).
- Calculate the volume: V = (dry mass − submerged mass) / 1.0 g/cm³
- Calculate the density: ρ = dry mass / V
- Compare the result with the target figures in the table above.
A deviation of more than ± 0.5 g/cm³ from the target is a serious warning sign. For coins it is worth adding a coin dimensions check (diameter and thickness).
Quick recap
Specific gravity is one of the oldest and most dependable physical methods of material testing — Archimedes is said to have grasped the underlying idea as far back as the 3rd century BC. For precious-metal owners the message is simple: combine a scale with a glass of water and you can expose many fakes at no cost and without a laboratory.