COLOURED GEMSTONES

The colour of a gem largely depends on how it absorbs or reflects light energy. When white light strikes a gem, some of the spectral colours are absorbed in preference to others (preferential absorption). Those that are not absorbed pass through the gem or are reflected back, giving the gem its colour.

Some gemstones have a characteristic absorption spectrum that, with the use of a gemological instrument called a spectroscope, can be used to assist identification, or for example to distinguish between two gemstones with a similar appearance but different spectra, such as ruby and garnet. Through the spectroscope, the absorption spectrum of the gemstone looks like an incomplete rain­bow, with black lines or bands replacing some of the colour. The lines and bands signify the energies with wavelengths that corre­spond to those colours that have been absorbed; the remaining energies give the gem its colour.

Idiochromatic And Allochromatic Gemstones

Where the colour of a gemstone is caused by elements that are an essential part of the chemical composition, the gem is termed idiochromatic (from idio meaning same and chroma meaning colour), for example the green colour of peridot is due to iron (Fe), an essential part of its composition (FeS2).

Most gemstones are allochromatic (from allo meaning other) and are coloured by small amounts of other elements or impurities called trace elements that are not an essential part of the chemical composition. The most common trace elements are the metals chromium, vanadium, iron, titanium, copper, and manganese. The bright green of emerald and demantoid garnet is caused by chromi­um, which also gives the bright red of ruby. sapphires are coloured blue by iron and titanium, or green, yellow or brown by iron. Man­ganese colours spessartine-garnet orange and rhodonite pink. Heat­ing, irradiation, and other means of altering, enhancing of destroying colour, are more likely to be effective on allochromatic gems than idiochromatic gems.

PARTI-COLOURS

During and after gemstone growth, changes in the surroundings may result in different trace elements being available for incorporation into the gemstone. These may result in a change in colour, colour banding, or patches of colour. When one part of a gemstone is a different colour to another part, it is called parti-coloured. Watermelon tourma­line, with its pink interior and green outer rim, is an excellent example of a parti-coloured gemstone, others may have more than two differ­ent colours.

Pleochroic (Dichroic And Trichroic) And The Dichroscope

A gemstone may also appear different colours or shades of colour when viewed from different directions (pleochroic). This is as a result of the way light travels through the crystal structure of the gemstone. When a gem shows two different colours or shades of colour, such as in some rubies, the gem is described as dichroic. Trichroic gem-stones such as iolite (cordierite) and tourmaline show three colours or shades of colour.

To see the different colours you have to view the gemstone from one direction, remember the colour, and then turn the stone and compare the image with the colour when viewed from the new direction. Slight changes in colour can be difficult to recognize. The dichroscope, a handheld gemological instrument with a polar-oid filter and similar in size to a hand lens (loupe), can be used to view two colours of a pleochroic gem at the same time enabling comparison to be made more easily.

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