Chemical Composition Crystal Structure Optical Characteristics Specific Gravity Durability Back to Gemology Pages. . . In general, larger crystals were formed in areas of slow cooling of molten rock, and smaller crystals in areas of more rapid cooling. Crystal systems Cubic Crystals in the cubic, or isometric, system have three mutually perpendicular axes of equal length.
Gemstones occurring in cubic crystal forms include diamond, the garnets, pyrite, and spinel.
Hexagonal Crystals in the hexagonal system have four axes, three of which are of equal length and intersect at 60 degree angles within a plane, and the fourth of which is perpendicular to the plane of the other three. Gemstones occurring in hexagonal crystals include beryl, corundum, quartz, and tourmaline. Tetragonal Tetragonal crystals have three axes intersecting at 90 degree angles, two of which are of equal length. Examples include zircon, rutile, and scapolite. Orthorhombic Orthorhombic crystals have three axes at 90 degree angles , all of which have different lengths.