Solid substances fall into two general classes, crystalline and amorphous. Those whose atoms show long-range order, like the squares on a chessboard or the loops in a chain-link fence, are crystalline; those whose atoms are arranged in no particular, repeating pattern are amorphous. Naturally occurring amorphous solids are also termed mineraloids.

Amorphous solids are made of the same elements that produce crystalline solids, often mixed in the same ratios. For example, pure silicon dioxide (silica; SiO₂) occurs both in a crystalline form (e.g., quartz); and in an amorphous form (e.g., glass). The difference between the two forms is one of atomic-level organization. Given sufficient time, as when precipitating atom by atom from a hydrothermal solution or solidifying slowly from a pure melt, silicon and oxygen atoms assume an orderly, crystalline arrangement because it is a lower-energy state and therefore more stable, as a pencil lying on...