Most Minerals are, as we say, Crystalline. Various Atoms, the basic constituents of all matter, are arranged geometrically in an orderly atomic structure, in so-called Lattices. These Lattices are the very reason for the development of Crystal forms. There are a few exceptions to this structure for example Amber, Jet, Obsidian, Tektites, Opal and ‘Metamict’ Gems are non-crystalline and are termed amorphous. more

The Seven Crystal Systems more

Occasionally minerals grow in open cavities, in rocks and veins, with no limitations to spatial expansion. In these rare cases crystals develop flat faces, resulting in distinct Crystal Forms. Basic geometry classifies these Forms into seven different crystal systems that are defined by Symmetry and Angle elements.