quartz]] has atoms in a near-perfect periodic arrangement; a polycrystal is composed of many microscopic crystals (called " crystallites" or "grains"); and an amorphous solid (such as glass) has no periodic arrangement even microscopically.]] A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macroscopic single crystals are usually identifiable by their geometrical shape, consisting of flat faces with specific, characteristic orientations. The scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification. The word crystal derives from the Ancient Greek word (), meaning both " ice" and " rock crystal", κρύσταλλος, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus Digital Library from (), "icy cold, frost". κρύος, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus Digital Library Examples of large crystals include snowflakes, diamonds, and table salt. Most inorganic solids are not crystals but polycrystals, i.e. many microscopic crystals fused together into a single solid. Examples of polycrystals include most metals, rocks, ceramics, and ice. A third category of solids is amorphous solids, where the atoms have no periodic structure whatsoever. Examples of amorphous solids include glass, wax, and many plastics. Crystals are often used in pseudoscientific practices such as crystal therapy, and, along with gemstones, are sometimes associated with spellwork in Wiccan beliefs and related religious movements.Regal, Brian. (2009). Pseudoscience: A Critical Encyclopedia. Greenwood. p. 51.