Amethyst: the purple variety of quartz, historically reserved for royalty and clergy. Its colour is caused by iron impurities and natural irradiation.
Citrine: the golden-yellow to orange variety of quartz.
Ametrine: a naturally bi-coloured quartz displaying zones of both purple amethyst and yellow citrine. Its sole commercial source is the Anah� mine in Bolivia, where specific hydrothermal conditions produced the two colours in tandem.
Prasiolite: a pale leek-green variety of quartz, typically produced by the careful heating of specific types of amethyst; treatment is disclosed at the point of sale.
