So what makes coloured gemstones coloured?
I think it’s pretty magical how the Earth creates such a diverse range of coloured gems, but what makes them so brightly and individually coloured?
Most gems are created deep underground when molten minerals cool and solidify under great pressure - this happens over thousands of years, and the minerals are then forced over the millennia towards the surface through volcanic or tectonic activity.
Some gem quality minerals, including quartz, corundum, diamond and beryl are transparent in their purest form. These minerals tend to have a simple crystal structure and don’t contain impurities or inclusions.
But as we know, gemstones and diamonds can be all the colours under the sun and this is due to the presence of various chemical impurities in their atomic structure.
Here are some of the chemicals which create colour in gemstones:
Nitrogen - which can give gemstones and diamonds a yellow or brown colour, and in some cases, pink or purple depending on the concentration and distribution of the impurities.
Boron - boron impurities can make gemstones a blue or purple colour.
Chromium - the presence of chromium can give gemstones, such as rubies and sapphires, a red or pink colour.
Iron - can make a gemstone yellow, orange, brown or green.
Copper - gives gems, such as turquoise and Paraiba Tourmaline, a blue or green colour.
Titanium - gemstones like sapphires can look yellow, orange, or pink if titanium is present.
Manganese - manganese impurities can give gemstones, such as spinel and morganite a pink or purple colour.
Iron and Titanium - In the case of blue sapphires, the blue colour is caused by the presence of iron and titanium impurities in the crystal structure of the gemstone. These impurities absorb certain wavelengths of light, causing the gemstone to appear blue to the human eye.
Chromium or Vanadium - The green colour of an emerald is caused by the presence of trace amounts of chromium and/or vanadium
The colour of a gemstone will depend on the type and concentration of the chemicals present, as well as the size and shape of the stone. In the case of coloured gemstones, a highly saturated colour is usually more highly prized than a subtly tinted one.
Sometimes the mineral will form with more than one chemical inside and it will end up as a multicoloured stone, a great example of this is Watermelon Tourmaline which contains green, clear and red bands and is usually cut to maximise the three colours in each gemstone.
There are also many coloured minerals that are opaque when they form, such as hematite, magnetite, and pyrite.
Treatments to enhance the colours in coloured gemstones:
It is a common practice to heat certain gems to enhance their colour and clarity. For example, brown or yellow sapphires are often heated in order to remove impurities or alter the crystal structure of the gemstone, resulting in a more desirable colour.
During the heat treatment process, the sapphire is placed in a special oven or furnace and heated to a temperature of around 1800-2000°C. The heat causes the molecules in the sapphire to vibrate and break apart, which removes or rearranges the undesirable brownish impurities. The sapphire is then cooled slowly in order to allow the crystal structure to reform in a more desirable way.
After heat treatment, browny tinged sapphires may appear more blue or green in colour. The exact shade of the sapphire will depend on the specific impurities present in the gemstone and the temperature and duration of the heat treatment. Heat treatment is a common and accepted practice in the gemstone industry, but it is important to note that heated sapphires have to be reported and sold as treated sapphires.
Have you found out what makes your favourite gem colour? Let me know in the comments below!
