Amethyst is the most valued member of the quartz family. It displays a range of shades from deep purple, light lilac, lavender and mauve. Amethyst may exhibit one or both secondary hues of red and blue. Top quality amethyst is considered a deep medium purple with rose-colored flashes. The ideal grade is called "Deep Siberian" and has a primary purple hue of around 75–80%, with 15–20% blue and (depending on the light source) red secondary hues.
Green quartz is sometimes called green amethyst. Other names for green quartz are prasiolite, vermarine or lime citrine. Amethyst generally becomes yellow when exposed to heat. Much of the citrine, cairngorm and yellow quartz is said to be "burnt amethyst".
In the 19th century, the color of amethyst was attributed to the presence of manganese. Recent studies have shown a complex interplay of iron and aluminum is responsible for the color. When manganese is present in clear quartz, amethyst is produced. The amount of iron contained in the specimen accounts for the depth of purple color. The amethyst has its hardness, moderate refraction and weight in common with the other quartzes, but the crystal structure is different, and most unconventional. The construction is stratified, as a result of which areas and lamellae of varying color intensity often come about. This explains why there are relatively few large cut amethysts of an evenly distributed dark color.
Amethyst is produced in abundance from the Brazilian state of Minas Gerais where it occurs in large geodes (“amethyst-grotto”) within volcanic rocks. Brazilian state Rio Grande do Sul is a large world producer exceeding in quantities of Amethyst. Most of the material from Brazil is light-colored and tender purple.