In optical microscopy, a plate used to determine the optical properties of minerals. Quartz, mica, and gypsum are the common minerals used to determine the slow and fast vibration directions that relate to the two refractive indices of an anisotropic mineral. The terms ‘length-fast’ and ‘length-slow’ may then be assigned to a given mineral for identification purposes. A wedge of quartz (quartz wedge) is used to determine the order of interference colour exhibited by a mineral.