A solid with a structure that exhibits a basically symmetrical and geometrical arrangement. A crystal
may already possess this structure, or it may acquire it through mechanical means. More than 50 chemical substances are important to the optical industry in crystal form. Large single crystals often are used because of their transparency in different spectral regions. However, as some single crystals are very brittle and liable to split under strain, attempts have been made to grind them very fine and compress the powder under heat and high pressure into the desired form as a "polycrystal.'' The optical properties of polycrystals are identical with those of a single crystal, but the material is incomparably harder and better able to withstand rough treatment. Some crystals that are valuable in the infrared are soluble in water, such as rock salt and cesium bromide. Most natural crystals are birefringent, one of the most birefringent being calcite, which is used in making polarizing prisms. Natural quartz is slightly birefringent, but fused quartz is not.