Quartz n. : A form of silica, or silicon dioxide (SiO2), occurring in hexagonal crystals, which are commonly colorless and transparent, but sometimes also yellow, brown, purple, green, and of other colors; also in cryptocrystalline massive forms varying in color and degree of transparency, being sometimes opaque.

Example:
Why do they use synthectic Quartz when making the Quartz for watches?

Taxonomy
quartz, Citrine, Rose quartz, quartz crystals, varieties, crystals, mineral, Amethyst, opaque, Varieties of Quartz, Smoky quartz, SiO2, Milky quartz, National Museum of American History, Quarz, Lemelson Center, colored varieties, Agate.

Defination from the web:
quartz is that of macrocrystalline (individual crystals visible to the unaided eye) and the ... Most commercial citrine is in fact artificially heated amethyst or smoky quartz. ...