Other linear polarizers exploit the birefringent properties of crystals such as quartz and calcite. Polarized irradiation will make an azo-material anisotropic and therefore optically birefringent and dichroic. Some proteins are also birefringent, exhibiting form birefringence. The birefringent material is placed between crossed polarizers. These optically anisotropic materials are said to be birefringent (or birefractive). For this reason, birefringent materials with three distinct refractive indices are called "biaxial". The compound crystallizes as tetragonal birefringent crystals. Indeed, thin slabs of birefringent crystals are commonly used to manipulate the light polarization. The birefringent crystal alters the character of the polarization. The former are easily confused with Glan-type birefringent polarizers. Media that have different indexes of refraction for different polarization modes are called "birefringent". Using polarized light the characteristic Maltese cross shape of birefringent crystalline pigment deposits is found. Hydroxyapatite crystals are small and negatively birefringent. Polarized light microscopy is commonly used in biological tissue, as many biological materials are birefringent. The vacuum itself is polarized, becoming strongly birefringent, like a calcite crystal.