Gemstones give very beautiful colour variety which attracts to the eyes of buyers. If people like the particular gemstones, it is because of their colour. Specific colour is known as gem colour. So, how the gemstones get their specific colour and how they turn into that particular colour? Here are some points discussed under below:-
What Causes Gemstone Color?
There are number of gemstones with great variety of colours. Each has its own colour characteristics. The causes of colours in the gemstones are basically from chemical composition of the gemstones. This should be kept in mind that many minerals have coloureless form in their pure form. Inclusion of impurities in their structure leads to their colours.
The transitional metal group have such kind of elements copper, chromium, iron, gold, nickel and so on. Then, there are some rare earth series having elements such as gadolinium, erbium and yttrium. The spectra not adsorbed by the gems is transmitted or reflected back to the viewer. We see the wave lengths of light and process them as colour. The net result of the wavelengths that pass through the gem, gives the final color to the gemstone. In order to see color, we need at least 3 variables:
- Light
- A substance
- Vision (the eye)
White is consisted on spectral colours which can be seen in rainbow normally. The colours of rainbow are red, yellow, orange, gree, indigo, blue and ciolet. Every colour among these rainbow colours travel at particular wavelength. For example, for red colour, it is 700mm and for violet colour, it is 400 mm. Each carries particular amount of energy.
When a substance catches all the spectral "rainbow" colours except red, the residual colour is red. So the result will be in red colour. Similarly, if substance absorbs all except red and blue, the residual colour ( net result) is purple colour. There are approximately 16 million of combination that can produce colour. Light energy to heat is transformed by the result of absorption of light. White colour does not absorb light. It is good in use to keep heat away.
It is not necessary to have presence of transitional metal for the origin of the colour in gems. There is other factor of absorption of wavelengths of visible light, the transfer of electrons between ions in gemstone structure. The absence of an ion in a specific location in the structure, or the presence of a foreign non-transition metal ion, can also lead to colouration, as can simple diffraction of light through the crystal’s structure.
Gems with having impurities which causes colour are called allochromatic. In case of having purity, they will be colourless. There is another condition for causing a colour which is called idiohromatic in which gemstones have such elements which are necessary for chemical structure. There are two type of coloring agents as a chemical structure or impurities.
- Idiochromatic - minerals get colours by an element included in their chemical composition or chemical formula such as Malachite.
- Allochromatic - Colours happens due to impurities and not part of their chemical compositions such as Emerald green emerald, pink morganite, and blue to blue-green aquamarine
Some elements's Role in colour variety:-
| Titanium | Blue |
| Vanadium | Green / Color change |
| Chromium | Red - Green |
| Manganese | Pink |
| Iron | Red - Green - Yellow |
| Cobalt | Blue |
| Nickel | Green |
| Copper | Green - Blue |
List of Gemstones with colour reasons
Aquamarine is variety of beryl. The iron in the gem caused pale colour.
Emerald green color due to the presence of both iron and titanium.
Garnet deep colour is from iron.
Peridot is usually found in volcanos. This is form of Olivine. The yellow-green color comes from iron.
Ruby if form of Corundum. The colour is normally due to Chromium. The colour of ruby is pink to red normally
Turquoise, the opaque mineral, gets its colour from Copper
Sapphire is variety of Corundum . In fact corundum in any colour except red is called sapphire. Here is Purple sapphire is from Kashmir, Pakistan. The blue is from Madagascar. The colour in sapphire is from iron and titanium.
