Amethyst is the purple variety of quartz. Iron in the crystal, together with natural radiation, causes its color.
In more detail
Amethyst is 7 on the Mohs scale and has good toughness. It often shows color zoning, which you see best table-down on a white surface. Long exposure to strong light can fade its color, and sudden temperature changes can crack it. Heat can turn it yellow, and most citrine on the market is made this way.
On a grading report
Labs identify it as (natural) quartz, variety amethyst. Laboratory-grown amethyst is named as such.
Related terms
- Quartz: Quartz is the mineral species of amethyst and citrine. It is silicon dioxide in a trigonal crystal.
- Citrine: Citrine is the pale yellow to brownish orange variety of quartz.
- Ametrine: Ametrine is quartz that is part amethyst and part citrine: purple and yellow zones in one crystal.
- Color zoning: Color zoning is uneven color in a gem: bands or patches of deeper and paler color.
- Stability: Stability is a gem's resistance to damage from heat, light and chemicals.
- Hydrothermal growth: Hydrothermal growth makes synthetic crystals in a hot water solution under high pressure.