CRYSTAL SHAPES

CRYSTAL SHAPES

MOST MINERAL GEMSTONES are crystalline, with their atoms arranged in regular and symmetrical patterns, like a lattice; a few are amorphous, with no or only a weak crystal structure. Crystalline minerals may consist of a single crystal, or of many in a group. Polycrystalline minerals are made up of many, usually small, crystals; in cryptocrystalline minerals the crystals are too small to see without the aid of a microscope. Crystalline minerals are made up of a number of flat surfaces called faces; the orientation of these faces defines the overall shape, which is known as the "habit". Some minerals have a single, characteristic habit, such as pyramidal or prismatic; others may have several. A lump of crystalline mineral without a definite habit is called massive. Amorphous gem- stones, like obsidian and tektites, have an irregular shape. Examples of common habits are shown right.

              

CRYSTAL SYSTEMS

Crystals are classified into seven different systems, according to the "minimum symmetry" of their faces. This depends on a crystal's "axes of symmetry"- imaginary lines (shown in black in the artworks on this page) around which a crystal may rotate and still show identical aspects. The number of times the same aspect may be seen - in one 360-degree rotation around an axis - defines that axis as two-fold, three-fold, etc., up to six.