Specific Gravity

Density and specific gravity are closely related mass-dependent properties and among the most useful in mineral identification. Density (ρ) is defined as mass per volume. ^[1] The usual units are grams per cubic centimetre (g/cm³). Specific gravity (G) is the ratio of a mineral’s density to the density of water at 4°C. ^[1] Because it is a ratio of two densities, specific gravity is unitless. And because the density of water at 4°C is essentially 1 g/cm³, the numerical value of specific gravity is the same as the density expressed in grams per cubic centimetre - a quartz sample with a density of 2.65 g/cm³ has a specific gravity of 2.65. This numerical equivalence makes specific gravity easy to apply in practice.

Why It Matters for Identification

Specific gravity is controlled by two things: the chemical composition of the mineral and the way in which its atoms are packed and bonded together. ^[1] Both of these are fixed properties of a given mineral species, which means specific gravity is a reliable and consistent diagnostic property. A mineral with high specific gravity feels noticeably heavier in the hand than one with low specific gravity at the same apparent volume, and this contrast alone can be diagnostic before any formal test is applied.

Field Estimation

A rough estimate of specific gravity can be made simply by hefting a sample in one hand and comparing it to a sample of known specific gravity in the other. ^[1] Quartz, with G = 2.65, is the standard reference for this test because it is common, easily obtained, and sits near the middle of the range for common rock-forming minerals. A mineral that feels noticeably heavier than quartz for the same apparent volume has a specific gravity well above 2.65; one that feels lighter is below. This is not a precise measurement, but it is fast and requires no equipment.

References

1. Nesse, W. D. (2017). Introduction to Mineralogy, 3rd ed. Oxford University Press.

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