Cover image: root replacement pipe crystal boulder opal from Australia. 11.8 × 6.0 x 3.8 mm and 1.62 carats (credit: Tom Goskar).
My mind regularly imagines its way through the microscopic worlds of gems, minerals and rocks we can’t see. The jumping atoms causing interesting colours, the crystal lattices literally bending light in different directions, and so on. The rainbow-type effects we observe in gemstones can be caused by all different types of diffraction: the splitting and bending of white (visible) light into its spectral (rainbow) colours. Also known as iridescence, it can be caused by a thin film inclusion causing a patch of rainbow inside a quartz crystal or the many layers of feldspars causing interference, refraction and diffraction in a piece of labradorite.
Australian flagstone to pinfire
And it can be the play of colour we associate most with precious opal. Play of colour occurs when light is reflected and refracted by the gemstone as you turn it, flashing from red to orange, yellow and green, and blue and violet. There are so many names for the different patterns precious opal–or opale noble–can produce–whether as precious black opal, crystal opal (opal without a matrix) white opal or boulder opal where the mineralisation has taken place in gaps left in a rock and even in the gaps left by the roots of ancient trees and fossils. Fire opal – the orange to red translucent to transparent material mainly found in Mexico – might exhibit a subtle play of colour. Check out Black Opal Direct’s guide to identifying different opal patterns. You’ll find flagstone, cloverleaf, ribbon, pinfire and broad flash among many others.
Precious opal can be found in a few locations around the world but the biggest producer of the best quality precious opal, particularly the most sought after, black opal, is the eastern part of Queensland, Australia around regions such as Lightning Ridge and Coober Pedy. It is no surprise that precious opal is Australia’s national gemstone. Hunting for opal is a vocation for those who do it. It requires a lot of patience, money and a huge amount of luck. This story about black opal hunting by Al Jazeera talks to those who get addicted and the communities that form around the pursuit. The other well-known locality for precious opal, particularly crystal opal, is Ethiopia, however quality can vary and there is a bit of controversy around a treatment called ‘smoking’ where crystal opal is smoked (like a piece of cheese or meat) to darken the translucent mineral and make a darker background for the play of colour. If this is declared and priced accordingly there isn’t a problem but it does get passed off as black opal which is wrong. Do not confuse this material with synthetic stimulant opal which is a stunning material, resembling precious opal but not chemically identical.
Common and precious
Precious opal is the traditional birthstone for October, and this October I have seen so many rainbows as our weather slowly slides from Autumn into winter, that I thought it would be nice to take a closer look at the very particular molecular structure of opal and explain the cause of its rainbows. Opals are amorphous gemstones meaning they do not conform to a strict crystal lattice geometry like quartz, diamonds and tourmaline.
Opal is a hydrated silica (SiO2.nH20). Silica is one of the most abundant minerals on earth but the particular geological story of precious opal is what has contributed to its astonishing colours. In fact due to its amorphous structure it is more correctly known as a mineraloid. It’s a light gemstone, not very dense, with a hardness of 5.5 to 6. Opals can contain up to 20% water so particular care against thermal shock and drying out must be observed when working opal, setting it and storing it to prevent crazing and cracking. I might pause here to say there is also such as thing as common opal. Common opal is chemically identical to precious opal but has formed without play of colour. As the name suggests, common opal is found in more widespread locations, including here in Britain (see Cornwall’s china clay region). Colours range from white to yellow to brown and from South America, especially Peru, you might get pink and blue.
Sphere-forming ancient seas
Opal is formed in seams like other amorphous gemstones such as turquoise. Water seeped down through cracks and crannies in the earth’s crust forming a magic solution with silicon dioxide derived from sandstone and other sedimentary host rocks. As the water evaporates a silica deposit remains and, layer upon layer, minute spheres of hydrated silica jostle and arrange themselves eventually leading to the creation of a seam of opal. Imagine the roots of a tree replaced by a hot fluid which eventually cools into a gel and then solidifies into multi-colour pipes and nodules.
I have adapted this explanation of the geological formation of opal from the National Opal Collection. During the Cretaceous period (65-140 million years ago) the island of Australia was home to an inland sea. Fine marine sands rich in silica were deposited around the shoreline. 30 million years ago (Tertiary period) this great sea dried up to form the great Artesian Basin. A period of harsh weathering caused the sandy sediments to release the soluble silica which eventually gelled and hardened as described above. It’s estimated that it took 5 million years for an opal to become 1cm thick. Some of the most stunning opals are formed as replacement minerals in decomposing fossils and shells like ammonites from the early Cretaceous period around 120 million years ago.
Peeking into the opal universe
During my gemmology training I got interested in the imagery that is produced from SEM (Scanning Electron Microscope) analysis of different materials. I had some vague familiarity with its use, and that of XRF and Raman Spectroscopy from the analysis conducted on archaeological material. In the collection of the Wellcome Collection of London, I found a lovely colourised image (electron micrograph) of opal produced from SEM. The image shows all of the silicate spheres that give opal its structure, and play of colour. It was produced by David Gregory and Debbie Marshall. As the name suggests SEM enables you to view molecules and even atoms. There is no scale on this image but the resolution is measured in nanometers–millionths of a millimetre. The physics of opal’s structure had remained a mystery until SEM analysis of opal’s unusual structure and arrangement of silica spheres. These spheres can aggregate together in particular orientations and the gaps between the spheres are what allows white (visible) light to bend (refract) and then split (diffract) into its spectral colours. Small gaps produce violets and blues while larger gaps produce reds and oranges as those wavelengths are reflected back through the layers of the material. The way which which the angle of incidence at which light hits the opal surface changes will a then produce those colour flashes we associate with this cosmic beauty.