Gems are those which receive their colour from the presence of impurities.
The array of beautiful colours we see in natural gemstones can be created by one of three events:
Firstly, it can be due to the inherent chemical makeup of the crystal; these gems are known as idiochromatic. Peridot for example is idiochromatic.
Secondly, a gem’s colour can be caused by the optical properties of the gem and its reflection from or just under its surface.
Thirdly, and most common, gems coloured by the presence of impurities are known as allochromatic. All allochromatic gemstones would be colourless without the presence of impurities. The main elements that add colour to gemstones are titanium, chromium, manganese, iron, cobalt, nickel, and copper. The presence of these elements in gems such as Sapphire, Beryl, Spinel and Quartz are the sources of their vibrant array of beautiful colours.
Take Corundum as an example. In its purest form it is colourless (White Sapphire). When traces of iron are present we see a gorgeous Yellow Sapphire, add a small amount of titanium alongside the iron and you have the famous Blue Sapphire and when chromium is added to Corundum it becomes a rich red Ruby.
Without the presence of impurities, transparent allochromatic gems are colourless and opaque gems are white. In the case of Quartz and Beryl, the colourless varieties are more common, thus actually making them less valuable than those with impurities. For example, compare the price of a colourless 2 carat Beryl (known as Goshenite), to an equivalent carat weight of light blue Beryl (Aquamarine), and the price increases substantially. Then take the colourless Beryl, add a trace of chromium or vanadium and the price explodes exponentially as the gem now becomes an Emerald.
However, other gems such as Tourmaline and Jade are rarely found colourless making them more valuable than their coloured equivalents. The allochromatic gemstone Spinel, available in almost every colour imaginable, has yet to be discovered colourless.
