Some of the effects related to absorption like colour, luminescence, pleochroism etc. have important applications in gemology.
- The color is the most obvious and conspicuous property of a large number of minerals.
- These mineral are distinguished by an extra-ordinary variety of colour, shades, richness and intensity.
- The colour of a gemstone is determined by:
- The light by which it is observed.
- The gemstone itself.
- The eye of the observer.
White light is composed of all the colours or wavelengths that make up the visible spectrum (VIBGYOR). When white light falls on a gemstone:
- Some wavelengths are fully absorbed.
- Some are totally transmitted.
- Some wavelengths are reflected back by the surface of the gem.
- The colour we see is due to the selective absorption of light and the wavelengths which are reflected back by the surface.
The term colour can be explained by at least three different aspects.
- Property of an object: Blue sapphire.
- Characteristic of light rays: As blue sapphire efficiently reflects blue light while absorbing light of other colours more or less completely.
- Sensations: The brains interpretation in which the eye perceives light selectively, reflected from sapphire, results in perception of blue.
Causes of Colour
There are a number of different causes of colour in gemstones.
- The best known cause of colour is that derived from transition metal compounds or impurities. This provides the colour of many of our minerals, gems and pigments.
- The transition elements are a group of elements in the periodic classification of elements which have the ability to absorb energy from the light rays as they pass through the crystal, giving rise to the colour in a gemstone.
- These transition elements are Titanium (Ti), Vanadium (V), Chromium (Cr), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni) and Copper (Cu), and are the most important colouring agents in minerals and gems.
- Idiochromatic Gemstones (self colored): In these stones, the transition metal is an essential ingredient of the chemical composition e.g. Manganese in Rhodonite and Rhodochrosite produces a red color.
- Allochromatic Gemstones (other colored): In these stones the transition metal is present as an impurity and not as an essential ingredient of the chemical composition e.g. chromium produces the red colour in Ruby and the green of Emerald.
- Pseudochromatic Gemstones(false colored): This is a general term which is applied to causes of color due to:
- Lattice Imperfections: It results a colour centre or a defect hole. This occurs when ions of one element are present in excess or less than the number required by the chemical formula e.g. amethyst, smoky quartz and fluorspar.
- Semi-conductors: It includes diamond and pyrite. In the case of diamond, the presence of nitrogen and boron produces the yellow and blue colours respectively.
- Physical optics: The colour in this case may be due to interference of light which causes irridescence in Iris quartz and play of colour in labradorite. Diffraction causes play of color in opal and dispersion produces the fire in gems.
- In some gems, the colour may be due to the presence of coloured inclusions e.g. some varieties of chalcedony, quartz and calcite.
- In some varieties of gemstones even within a single crystal, the colour may vary. A good example is that of parti-coloured tourmaline. A single crystal may show three colours – red, almost colourless and green at the other extreme, as a result of which this is often called watermelon tourmaline.