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Gemstones














Colour investigation: Opal

The quality of colour can be dissected into vibrancy, evenness, pattern and, in some cases, uniqueness; however, what on earth causes it? This series is dedicated to exploring the different causes of colour in gemstone minerals. STACEY LIM reports.

First off the block this month is opal. Famously Australian and captivatingly beautiful, precious opal has an alluring display of colour that can be attributed to its unique internal structure.

Spectral flashes that appear to dance on the gemstone’s surface are aptly described as ‘play-of-colour’. This phenomenon observed in precious opal has no reference to the body colour of the gemstone but rather to its sheen, and is due to a reflection of light from beneath the gemstone’s surface.

Opal is a form of hydrated silica containing up to 21 per cent water existing as free water within the opal or bonded to other atoms in the structure. Millenia ago, Australia’s now dry outback was drenched from seasonal rainfall. Water carrying weathered particles of silica in saturated solutions soaked through the sandstone ground rock, infilling cracks and cavities left behind from geological movement deep in the ancient bedrock.

The landscape dried as the climate shifted, evaporating water and leaving the silica particles to merge together into a solid state in the form of microscopic silica spheres. Essentially amorphous, opals are without a clearly-defined external form. Rather, they take on the shape of the space in which they are formed.

When conditions are ideal, the silica spheres are uniform in size and appear in a stacked structure that is orderly and symmetrical. Small voids occur between the spheres to create an environment where white light can be reflected and diffracted as it enters the opal into different wavelengths. This results in playful, kaleidoscopic flares known as opal’s play-of-colour.

Without this perfect formation, play-of-colour cannot occur and the resulting material is called common opal or ‘potch’ – an abundant, translucent to opaque, single-coloured material. This potch is mostly used for backings of composite gemstones unless it is characterised by attractive colour or interesting inclusions, such as dendrites.

The body colour of opal is caused by trace elements that are present during formation. Iron and titanium oxides typically cause brown-black tones while nickel and chromium can produce green. Other colours include white, yellow, pink, red and blue.

Although opal’s play-of-colour means the gemstone can produce every hue of the rainbow, it is not always the case. Silica spheres of different sizes result in different colours and violet, blue and green are the most commonly occurring. Rarer and most enticing is red or any opal displaying a full spectral range.

In the end, it is the hue, brightness, pattern and directionality of an opal’s play-of-colour that determines the gemstone’s quality and desirability. Poor-quality opal is a relatively abundant mineral found in many locations around the world; however, finding material that displays play-of-colour is rare and this distinctive attribute has made precious opal a highly-sought gemstone, particularly in the international market place.












ABOUT THE AUTHOR
Stacey Lim

Contributor • Registered GAA Gemmologist & Valuer


Stacey Lim FGAA BA Design, is a qualified gemmologist and gemmology teacher/assistant. She is a jewellery designer, marketing manager and passionate communicator on gemmology. For information on gemstones, visit: gem.org.au









Wednesday, 23 May, 2018 11:09am
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