polycrystal, any solid object composed of randomly oriented crystalline regions, called crystallites, especially as distinguished from a single crystal (q.v.). Polycrystalline materials result when a substance solidifies rapidly; crystallization commences at many sites (see nucleation), and the structurally ordered regions growing from each site intersect each other. The random arrangement of the boundaries between individual crystallites in a polycrystal causes them to scatter a beam of light instead of reflecting or refracting it uniformly, so that even colourless polycrystals are opaque. Other mechanical, electrical, or magnetic properties of single crystals are similarly altered by the absence of long-range order in polycrystals.