Crystal structure (7 + Amorphous & Organic)
Gemstones may be formed in single or multiple discrete crystals (such as diamond), in massive collections of microscopic crystals (cryptocrystalline ) (such as chalcedony), or in amorphous (non-crystalline) masses (such as opal). In general, larger crystals were formed in areas of slow cooling of molten rock, and smaller crystals in areas of more rapid cooling. There are several classes of crystal structure based on symmetry of the resulting crystals, and there are also noncrystalline (amorphous) minerals used as gem materials. In addition, there are some organic materials (such as shell and bone) that have been used traditionally as gem materials.
Crystals in the cubic, or isometric, system have three mutually perpendicular axes of equal length. Common forms in the cubic system are the tetrahedron (4 faces), the cube (6 faces), the octahedron (8 faces), the dodecadehedron (12 faces), the trapezohedron (24 faces), and the hexoctahedron (48 faces). Gemstones occurring in cubic crystal forms include diamond, the garnets, pyrite, and spinel.
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. Some crystallographers further identify two subdivisions of hexagonal crystals: trigonal (corundum) and rhombohedral (quartz).
Tetragonal crystals have three axes intersecting at 90 degree angles, two of which are of equal length. Examples include zircon, rutile, and scapolite.
Orthorhombic crystals have three axes at 90 degree angles , all of which have different lengths. A typical example is topaz.
Monoclinic crystals have three axes of unequal length, two of which intersect at an angle other than 90 degrees, and both perpendicular to the third. Jadeite and nephrite are common examples.
Triclinic crystals have three axes, all of unequal length and intersecting at angles other than 90 degrees. Examples include labradorite and microcline feldspar.