The specific gravity of a diamond is a fundamental physical constant that plays a significant role in gemology, providing a reliable measure for identification. This property, often abbreviated as SG, is a standardized characteristic for all minerals and gemstones, indicating how dense a material is relative to a common reference substance. Analyzing specific gravity helps gemologists classify unknown stones and differentiate a genuine diamond from its numerous imitations. This consistent measurement is a useful tool when visual characteristics are not enough to confirm a stone’s identity.
Understanding Specific Gravity
Specific gravity is a measurable quantity that represents a ratio, comparing the density of a substance to the density of water. It is a dimensionless number because it is the result of dividing two densities measured in the same units. Water is the standard reference material, and its density is measured at a specific temperature, usually 4 degrees Celsius, where it is densest. The resulting specific gravity number simply tells you how many times heavier the material is than an equal volume of water.
This ratio provides a more stable and comparable value than simple density. To determine specific gravity, a simple calculation is performed: the density of the gem is divided by the density of the water. For example, a gem with an SG of 3.0 is three times heavier than the same volume of water. Since every gemstone has a characteristic chemical composition and crystal structure, its specific gravity remains a constant value, offering a predictable means of classification.
The Numerical Value of Diamond’s Specific Gravity
The specific gravity of a pure diamond is exceptionally consistent, typically falling within a narrow range of 3.50 to 3.53, with 3.52 being the commonly cited average. This specific number is a direct consequence of the diamond’s unique internal structure. Diamond is a crystalline form of pure carbon, where each atom is bonded to four others in a dense, three-dimensional tetrahedral lattice arrangement.
This highly ordered and tightly packed isometric crystal structure results in a high mass concentrated into a small volume, giving diamond its distinctive density. Slight variations in the specific gravity reading can occur due to minute trace impurities, such as nitrogen, or small inclusions within the stone. However, these factors rarely push the value outside of the established 3.50 to 3.53 range, which makes the number a highly dependable constant for identification.
Using Specific Gravity for Gem Identification
The practical application of specific gravity in gemology is primarily to distinguish genuine diamonds from diamond simulants. Gemologists can easily compare the measured SG of an unknown stone to the known constant for diamond to determine authenticity. The process most commonly used to find this value is called hydrostatic weighing, which is based on Archimedes’ Principle. This technique involves accurately weighing the stone in air and then weighing it again while fully immersed in water.
By using a specialized balance, the weight loss when submerged allows for the precise calculation of the stone’s specific gravity. This test is highly effective because most common simulants have significantly different SG values. For example, Cubic Zirconia (CZ), a popular diamond imitation, has a much higher specific gravity, generally ranging from 5.6 to 6.0. This means a CZ stone of the same size as a diamond would feel noticeably heavier.
Conversely, Moissanite, another common simulant, has a slightly lower SG, typically around 3.17 to 3.22, making it slightly lighter than a diamond. The marked difference in these specific gravity values provides gemologists with a simple, yet powerful, non-destructive test to confirm a diamond’s identity.