“Of all precious stones, it is Opal that presents the greatest difficulties of description, it displaying at once the piercing fire of Ruby, the purple brilliancy of Amethyst, and the sea-green of Emerald, the whole blended together and refulgent with a brightness that is quite incredible.”
Pliny the Elder (23-79 AD), Historia Naturalis
The name ‘Opal’ derives from the Latin ‘opalus,' which in turn stems from the Greek ‘opallios,' meaning ‘seeing a change.' Another possible Greek etymology is ‘opthalmios’ (gem of the eye), but the original source of the name is likely the Sanskrit ‘upala’ (precious stone), which lends credence to the belief that India was once a source of Opal for Ancient Rome.
Chemical composition of Opal
Opal’s structure is unique, and consists of minuscule spheres of silicon dioxide aligned in a pyramid-shaped grid. It’s the refraction of light through the spaces between these spheres that produces Opal’s characteristic and unique ‘play of color’ - the flashes of color that change with the angle of observation. Interestingly, Opal without ‘play of color’ such as Mexican Fire Opal has its silicon dioxide spheres more randomly arranged.
Opal is extracted from sedimentary rock, where it fills cavities or veins. Extraction from deposits, along with the selection and cleaning of raw Opal, is done by hand.
Today, 95% of Opal with play of color is sourced from a handful of Australian mining areas, including Andamooka (discovered in 1930), Coober Pedy (1915), Lightning Ridge (1902), Mintabie (1931), White Cliffs (1890) and the Queensland Boulder Opal fields (a vast area discovered in 1869 surrounding the town of Quilpie). The marvelous, bright colored Welo Opal is extracted in Ethiopia. Sources of Opal without play of color are located in Brazil, Peru, Mexico and Tanzania.
The history of Opal
A gemstone with as many varieties as Opal is surrounded by numerous legends and traditions. For a long time, it was used as a talisman, believed to bring fortune and success to the wearer. The Ancient Greeks believed that Opal allowed one to see the future, while the Ancient Romans saw it as a symbol of hope and purity. The Arabs believed that Opals had fallen from the sky with a lightning bolt, and thus could protect against lightning bolts, in addition to conveying invisibility.
For tribes from the Andamooka region in South Australia, the Opal was known as the ‘fire of the desert’ and was linked to creation myths. Their ancestral creator came down to earth on a great rainbow, which turned the rocks it touched into resplendent Opals, colored with the hues of a rainbow. In reality, we need to look to a huge inland sea and a resulting geological feature called the ‘Great Australian Basin’ to understand the origins of this gemstone. The majority of Australia’s Opal fields are located in the basin and were formed from the weathering of sandstone deposited over older host rock.
The mysterious Black Opal
Properties of Opal
Opal is well-loved for its unique play of color and flashes of light that change depending on the angle of observation. In addition to its multi-colored brilliance, Opal is also sought after for its stability - an important criterion when grading a gemstone that contains six percent water. A typical phenomenon is the so-called ‘opalescence,’ a gentle shining light that slips across the surface of the gemstone. The cause is interference; light diffuses through the subtle layers of the stone, creating this marvelous visual effect.
Varieties of Opal
Opals with color play are distinguished based on the host rock (also called ‘pots’ or ‘matrix’) on which Opal forms and their resulting transparency and body color (the base color on which Opal’s ‘play of color’ is visible).
Mexican Fire Opals
Black Opal has a black body color and may be opaque with some translucency, particularly when held to a strong light source. Gray Opal is a term seldom used, with its specimens typically being grouped in the ‘black,' ‘dark’ or ‘semi-black’ categories, but it is translucent to opaque with a gray body color.
White Opal, also called ‘Clear Opal’ is a popular and contemporary stone. It may be translucent or opaque, with a white body. Welo Opal displays an electric neon color and various patterns on a predominantly white body, although some examples are yellow or amber. Jelly Opal may be transparent or translucent, and due to the absence of host rock is either colorless or, viewed more optimistically, is called ‘Crystal Opal,' ‘White Crystal Opal,' ‘Dark Crystal Opal’ or ‘Black Crystal Opal.'
Opals with the matrix (host rock) included in the cut are called Boulder Opals; every Opal with host rock visible on the surface is called ‘Matrix Opal.'
Not all opals show a play of color, however. Fire Opal is a transparent to translucent crystal Opal with virtually no ‘play of color,' displaying oranges, reds, yellows and, since a recently discovered Brazilian deposit, also blues. Buriti Fire Opal is sourced in Brazil, and enchants with its reds and oranges, reminiscent of Mexican Fire Opal. Peruvian Opal is a rare variety that exhibits exquisite translucent blues, pinks and greens, also without play of color. Another variety without play of color is the translucent Yellow-Green Opal from Tanzania.
Care for Opal
Some examples may break due to loss of water; therefore, it is important to protect Opals from strong heat sources and particularly dry environments. Repeated temperature shifts can damage this gemstone.