Amethyst is a variety of quartz (silicon dioxide) with a violet/purple hue. A semi-precious stone that was worn by the ancient Greeks and Romans, it is the birthstone for February. It has the same hardness as quartz (around 7 on the Mohs scale) which makes it quite suitable for jewellery.
The variation in colour as a primary hue ranges from light violet through to a dark purple. Secondary hues are typically blue or red. The purple colour of amethyst is due to iron impurities in the quartz and irradiation.
Layers of stripes of colour are common in this stone. Usually the
person cutting the gem will have these layers parallel to the top face
of the crystal. This is done to try and ensure the colour when viewed
from above is even across the face. As the layers of the violet or
purple colour present are sometimes quite thin, it can make it a
difficult job for the
Non-synthetic or natural amethyst can exhibit dichroic properties, where changing the angle of view will produce a shift in the colour of reds and blues returned to the eye. Amethyst can lose this trait when heated and the overall hue will become yellow/orange or yellow/brown. Overexposure to bright light can also cause amethyst to fade resulting in lighter tomes of colour.
From the time of the ancient Egyptians amethyst has been used in many forms of jewellery including pendants, amulets and beads. Larger pieces of the crystal have been carved or engraved in a style known as ‘intaglios’. Today they are widely used in all manner of jewellery and cut in the range of styles common to rubies, sapphires and emeralds.
After the 18th century the discovery of large quantities of amethyst saw the gemstone reclassified from precious to semi-precious and its value lessened. As a semi-precious stone, it is more the colour than the carat weight that defines its value, unlike that of the precious gemstones.
Amethyst can be found in many places around the globe in veins, the largest being in Austria. This gem also is commonly found inside geodes, which are hollow rocks with a volcanic or sedimentary origin.
Amethysts can be synthetically created by doping a clear quartz crystal with ferric (iron) impurities and exposing it to x-rays or gamma-rays in a process called irradiation. By exposing it to heat, the effects of this irradiation can be cancelled to a degree and the colour will shift towards yellow/green.
Citrine, a yellow/orange quartz is commonly called ‘burnt amethyst’. Other varieties of quartz used in jewellery are known as agate, onyx, jasper, tiger’s eye, rose quartz and prasiolite. These are semi-precious stones and in the main tend to be translucent rather than transparent.