October Birthstone: Opal

Opals come in a variety of colours

Opals are most prized for their unique fiery play of colour, reflecting and refracting light into brilliant flashes of multiple colours. The true source of this play of colour was mistakenly attributed to Adularescence until the invention of the scanning electron microscope. Subsequent  analysis at 20 000 time magnification uncovered the true cause of the effect we know today as Opalescence.

Opal is hydrated silicon dioxide, containing from three to twenty percent water. This feature coupled with its low hardness of 5.5 - 6.5 on the Moh's scale, means that Opals require special care as a gemstone

Colours: Various
Hardness: 5.5 - 6.5
Refractive Index: 1.37 - 1.52
Specific Gravity: 1.98 - 2.50
Crystal System: Amorphous
Enhancements: May be enhanced
Major Sources: Mexico, Australia, Tanzania, Zimbabwe, Brazil

Warnings for Care
Soft, brittle  very susceptible to physical damage / heat

Opal Facts
Opal has a distinguished history dating back to pre Greek, Indian Sanskrit times. The name is derived from  the Indian Sanskrit word for stones  "upala". Three groups of Opal exist, precious opal, fire opals and common opals.

Opal - What Causes the Colour
The body colour or matrix in Opal  can be white, colourless, pale yellow, pale red, brown, gray or black depending on the host rock and the level of impurities.  Internal diffraction of light causes  flashes of any color across the entire spectrum (opalescence). The physical structure of opal is unique, with tiny spheres of silicon dioxide having orientated themselves during the solidification of a mineraloid gel, into pyramid shaped grids interspersed with water molecules.  A combination of the size and spacing of the spheres and tiny natural faults between the grid layout, causes light to be split into its constituent spectrum of colours, which results in the characteristic play of color. While the diffraction and interference can express itself as every colour in the visible spectrum, each Opal is unique in terms of its internal structure and the colours observed. Precious above all other varieties displays multispectral hues that change with the angle of observation.

Precious Opals
Black Opal, is the "King of Opals" which is a precious opal with black matrix color. The high contrast between the matrix and the opalescence ensure this variety has the highest play of colour.  White Opal, is also considered a precious Opal with white or very light matrix colour.

Fire Opals
Fire Opals are names after their orange colour, they do not opalesce and are generally milky or turbid in appearance.

Common Opals
Common Opals are mostly opaque, and devoid of the characteristic play of colour seen in Precious Opals.

Purchasing Opal
When choosing an opal, the lay of colour, matrix color and degree of transparency are the most important criteria which determine the price of precious opals.

While popular Opals are delicate gemstones, prone to dehydration, cracking, scratching and fading. When not being worn it is suggested that they be kept in a humidified environment (60-70%) such as in a plastic bag with a pierce of damp cotton wool, or in a cigar humidor.  For jewellery pieces Opals are better suited for earrings, brooches, and pendants, than rings and bracelets.

Colour
The higher the play of colour and the darker the matrix (for contrast) the more expensive the stones. The exception is Harlequin Opals, where the play of light can be relatively simple but the pattern in important with strong geometrical pattern being sold at a premium

Lighting
Opal looks best viewed in natural especially in the softer early morning or late evening light where the play of colour is maximised.

Clarity
Precious Opal are acceptable in, opaque, transparent or  translucent grades of clarity. The latter are rare and command a price premium

Cut
Mostly round or oval cabochon cut. Fine quality fire opals are facetted, although they are turbid rather than clear.

Mineral Deposits
Historically the main deposits were in Czechoslovakia, however since the early 1900's Australia has been the pre eminent producer of most of the world precious Opal. Currently about  85 percent of the world's supply of white precious opals is mined from the famous deposits at Lightning Ridge, White Cliffs, Coober Peddy and Andamooka.  Other deposits are found in Mexico, Brazil, Guatemala, Honduras, Indonesia, Russia and the United States.

Opal - Common Treatments
Opal are typically impregnated with oils, waxes, or plastics as part  a fracture filling enhancement. This markedly improves the appearance of the stone although over time the effect can be lost, and if the wax or oil decolours then some of the play of colour may be lost.

Important & Famous Opals
There are numerous examples of large Opal in existence, three of the largest ever discovered are, the " Andamooka Desert Flame" (6843kg), Big Ben" (4000 carats), and the "Light of the World" (2250 carats,

Opal - The Legend
According to the Aborigine legends, the creator came down to Earth riding on a rainbow, in order to bring the message of peace to all the mankind. Where his foot touched the ground, the stones became alive and started sparkling in all the colours of the rainbow. That was the birth of the opals.

Taken from:  www.touchstonegems.co.za