The chemical, physical and optical properties of natural gemstones are shown through measurements of light dispersion, hardness, density or specific gravity, cleavage, fracture, tenacity, refractive index and luster. These properties vary depending on the type of natural gemstone. For example, amethyst has a specific gravity of between 2.6 to 2.7, a hardness of seven, a refractive index of between 1.54 to 1.55 and a chemical formula of SiO2.
Hardness is graded on the Mohs hardness scale from one to 10, with talc graded at one and diamonds at 10. The specific gravity is the relationship between the stone's weight and heaviness. Cleavage is a break due to directional weakness, while fracture is irregular surface cracks. Tenacity is the strength of the stone. For example, the natural gemstone aquamarine, a type of beryl, has a formula of Be3Al2(SiO3)6, a Strunz classification (based on chemical composition) of 09.CJ.05, a formula mass of 537.50, imperfect cleavage, a fracture of conchoidal (shell-like) to irregular, a brittle tenacity, a Mohs scale hardness of 7.5 to 8, a luster of vitreous to resinous, a streak of white, a diaphaneity of transparent to translucent, a specific gravity average of 2.76, a uniaxial optical property (with one optic axis) and a refractive index of either 1.564 to 1.595 or 1.568 to 1.602.