Amethyst is the purple form of crystalline quartz, and the birthstone for February.
Quartz is composed of silicon dioxide, which is also known as silica. It occurs in two main types, crystalline (made up of single large crystals), and polycrystalline (made up of lots of tiny individual crystals). The two forms look quite different and have very different properties.
Crystalline quartz is possibly the most widely recognised form of quartz. It occurs naturally in a variety of colours including rose quartz (pink), amethyst (purple), citrine (yellow), prasolite (green), smoky quartz (brown), and rock crystal (colourless). It is affordable, and widely used in gem-set jewellery.
Amethyst is one of the most valued varieties of quartz due to its rich purple colour. The colour is often not competely uniform throughout the stone, with colour variations often visible in larger stones. It is often seen in geodes (small cavities of rock lined with crystals), with the crystals showing a more intense purple colour at their tips compared to their colourless bases. Inclusions such as partially healed fractures are common.
Citrine is the yellow form of quartz, and its colour can vary from a pale yellow (sometimes known as lemon quartz) to a deep orange. It is often produced by heat-treating amethyst, as citrine occurring naturally is not common. (Conversely, amethyst can be produced by irradiating citrine!) A variation of citrine and amethyst which displays both purple and yellow colour zones is called ametrine.
Prasiolite is the green form of crystalline quartz. It can occur naturally but is rare, and much of the commerceally available prasiolite is formed by heat-treating certain types of amethyst. The commonly used term “green amethyst” is felt to be misleading by gemmologists, and should be avoided.
Rose quartz varies from pale to deep pink.
Smoky quartz colours vary from deep brown or black to smoky yellow. The colour can be altered by irradiation.
Certain specimens of crystalline quartz contain tiny needle-like inclusions. When these are orientated in a particular way one can see an optical effect called asterism, with rays of a star visible when the stone is moved below a light source.
Care of crystalline quartz:
Do not leave in strong light (especially sunlight) as colours may fade.