Amethyst
The purple variety of quartz — the most prized member of the quartz family, coloured by iron and irradiation.
Also known as: Purple quartz
beginner Volcanic geodes and hydrothermal veins
What it is
Amethyst is the violet-to-purple variety of quartz (silicon dioxide, SiO₂), its colour caused by iron impurities acted on by natural gamma irradiation. It is 7 on the Mohs scale, durable enough for everyday jewellery, and occurs in large, affordable crystals, which has made it the best-known quartz gem since antiquity.
Colour ranges from pale lilac to deep "Siberian" purple with red flashes. Strong heat can turn amethyst yellow or brown, producing citrine, so much commercial citrine is heat-treated amethyst. Major sources include Brazil, Uruguay, Zambia and Bolivia. Because supply is abundant, amethyst is valued primarily on the richness and evenness of its colour rather than rarity.