How Does Our Opal Classification System Work?

By Author: Ali C. Underwood
Total Articles: 39
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Where Do Opals Get Their Colour?
Opals can display all the colours of a rainbow in an iridescent and moving pattern of purple, blue, green, yellow, orange, pink, red and any other colour. This can take a number of captivating forms and this colour movement across the stone face is referred to as “play of colour”.

Origin

Opal owes its origin to silica filled waters; at low temperatures, silica gel precipitated to form nodules or layers of opal in veins, fissures and cavities of sedimentary and volcanic rocks.

Opal itself is the amorphous kind of silica and chemically, nearer to quartz. It contains about three percent to 21 percent water within the mineral structure. Opals which are of gemstone quality generally contain six percent to 10 percent water.

Kinds of Opals

Opals are of three kinds, Precious opal, Fire opal and Common opal. Precious opal can be identified by the characteristic “play of colour” when the stone is tilted and rotated. Fire opal is a translucent or transparent opal, which ranges in colour from yellow to orange and then to bright red. Common opal or “potch” ...
... as it is popularly known is generally white or colourless. However, if fine-grained impurities are present in the potch, then it can be of brown, gray, red or yellow colour. This comprises about 90 percent of all opal found in the market and has negligible commercial value.

Reason for Colour

Opals display their characteristic “play of colour” due to diffraction. Water percolates through earth if suitable conditions exist. Silicates present in the soil dissolve the water to form a saturated solution. When such water enters into a cavity, the silicates are deposited as tiny spheres. The precipitated silica sphere layers form a water mass similar to jelly, with a diffraction grating being produced when spheres are of even size and ordered. This arrangement of diffraction grating generates a rainbow-like, sparkling light play from the stone.

The main cause of play of colour is due to uniformity of those tiny spheres, with a diameter that is one-tenth the size of a micron. Common opal is created when these spheres have a random arrangement and shape. If the spheres are regular in shape and size, then there is an evident play of colour. The colours generated by regularly packed spheres, which make up the opal's internal structure, depend upon the sphere sizes and the spaces between them. If the stone is moved, light hits spheres from different angles and a change in colour is perceived.

Colour generated is dependent on the sphere size. Small spheres of less than 150 nm produce violets and blues, while bigger spheres less than 350 nm generate oranges and reds. The more uniform the spheres' sizes, the more brilliant and intense colours it will generate.