Quartz:

Optical properties:

Uniaxial positive; first order interference colors (gray to pale yellow); undulant extinction; clear in plane polarized light; conchoidal fracture; no visible cleavage or twinning. In intrusive rocks, quartz is typically a late-forming mineral and therefore interstitial to other minerals.

The photos on this page are under crossed polarizers: on the left above is strained quartz in a granite; on the right is a quartz phenocryst in rhyolitic glass, showing conchoidal fracture. The photos below show quartz grains in a sedimentary rock, conglomerate. On the left, Boehm lamellae in the grains indicate a high stress regime; the grains on the right show sutured grain boundaries and internal strain, indicating a probable metamorphic origin.

Occurrence:

Many intrusive rocks, such as granite, quartz monzonite, granodiorite, tonalite, etc; extrusive rocks such as rhyolite, dacite, etc., and many metamorphic and sedimentary rocks.