Crystal quartz (SiO2) is a very useful material because of its high UV, VIS and NIR transmittance, birefringence, ability to rotate plane polarized light, high damage threshold and resistance to scratching. The optical grade material is featured by highest possible transmittance throughout range 190 - 2900 nm, virtual freedom from bubbles and inclusions, conformity with Military striae Grade A.
Quartz is very hard (Moh's scale 7) and is very strong. It thus lends itself to the fabrication of very thin low order retardation plates. Unlike calcite or magnesium fluoride, quartz exhibits circular birefringence, and there is no unique direction (optic axis) down which ordinary and extraordinary beams propagate under one refractive index with the same velocity. Instead, the optic axis is the direction for which the two indices are closest: a beam propagates down it as two circularly polarized beams of opposite hand. This produces progressive optical rotation of an incident plane polarized beam; which effect is put to good use in rotators.
Quartz Crystal Properties:
Quartz Refractive Indices & Birefringence:
Transparency Curve of Quartz: