Specific Gravity
Also: Relative density, Density ratio, Spec. gravity
Specific gravity (also: relative density) indicates how much heavier a substance is compared with water (density 1.0 g/cm³) and is one of the most reliable metrics for the authenticity testing of precious metals.
Specific gravity (technical term: relative density) is a dimensionless quantity that describes the ratio of the density of a substance to the density of pure water at 4 °C (= 1.0 g/cm³). In practice the term is often used synonymously with the density in g/cm³, since the numerical value is identical. For precious metal buyers and testers the quantity is indispensable: a genuine gold bar feels noticeably heavier for the same size than a fake made of a lighter metal – the specific gravity reveals the deception.
Formula and Measurement
Density results from mass and volume:
ρ = m / V
ρ = density in g/cm³
m = mass in grams (dry weighing)
V = volume in cm³
For irregular objects (coins, broken gold) the volume is difficult to measure directly. The most elegant method is the Archimedean principle: you weigh the piece dry, then submerged in water. The difference equals the weight of the displaced water – from which the volume follows, and from that the density. Our Authenticity Test Calculator performs 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 | most important reference value |
| Tungsten | 19.25 | dangerous counterfeit metal |
| Palladium | 12.02 | noticeably lighter than gold |
| Silver (999) | 10.49 | easily measurable |
| Copper | 8.96 | base metal in alloys |
| Aluminium | 2.70 | immediately noticeably light |
The closeness of tungsten (19.25 g/cm³) to the gold value (19.32 g/cm³) makes tungsten counterfeits hard to expose by density testing alone – here additional methods such as ultrasound or X-ray fluorescence are required.
The Influence of Fineness on Density
Alloys deviate from the pure-metal values. The lower the fineness, the more strongly the alloying metal affects the overall density:
- 750 gold (18 carat): approx. 15.5–16.5 g/cm³ (depending on the alloying metal)
- 585 gold (14 carat): approx. 12.9–14.0 g/cm³
- 925 silver (sterling silver): approx. 10.36 g/cm³
Anyone wishing to calculate the melt value of scrap gold needs the fineness – and anyone who does not know the fineness can narrow it down via a combined density and weight measurement. The Coin Weight Checker automatically compares target and actual weight for common coins.
Practical Authenticity Testing
- Dry weighing with a fine scale (resolution 0.01 g or better).
- Submerged weighing in distilled water (thread or basket, no air bubbles).
- Calculate volume: V = (dry mass − underwater mass) / 1.0 g/cm³
- Calculate density: ρ = dry mass / V
- Compare the result with the target values in the table above.
A deviation of more than ± 0.5 g/cm³ from the target value is a serious warning sign. For coins, an additional coin dimensions check (diameter and thickness) is recommended.
In Brief
Specific gravity is one of the oldest and most reliable physical methods of material testing – Archimedes is said to have grasped the basic idea as early as the 3rd century BC. For precious metal owners the rule is: anyone who combines scales and a glass of water can expose many fakes free of charge and without a laboratory.