Common or potch opal, 6cm x 4cm Locality unknown
Opalised ammonite, 6,5 cm x 4,5 cm Madagascar
 
Crystal system:                Amorphous                                
Hardness:                        4,5 – 6,5
Density:                           1.9 – 2.3                                                    
Streak:                            White   
Cleavage:                         None                                          
Composition:                    Hydrated silica     SiO2 nH2

The word opal is adapted from the Roman term opalus, but the origin of this word is a matter of debate. Most modern references, however, suggest it is adapted from the Sanskrit (ancient Indo-Aryan language) word upala. Another claim is that the word is adapted from the Greek word, opillos. This word has two meanings, one is related to “seeing” and forms the basis of the English words like “opaque”, and the other is “other” as in “alias” and “alter”. It is claimed that apalus combines these uses, meaning to see change in colour. The argument for the Sanskrit origin is the strongest and the term first appeared in Roman references around 250 BC. At the time opal was valued above all other gems and was supplied by traders from the Bosphorus, who claimed the gems were being supplied from India. Okay! This concludes the history lesson for the day.

Opal is an amorphous form of silica related to quartz, and is therefore classified as a mineraloid and not a mineral. Between 3% and 21% of its total weight is made up of water, whereas quartz contains no water. One of the scientifically accepted standards defining a mineral is that a mineral must have a crystal structure, which opal lacks. Despite this almost all scientific references, including the well-known Dana’s System of Mineralogy, categorize opal together with true minerals. It only forms crystal shapes when it forms as pseudomorphs after another mineral.

During the Cretaceous period, some 135 to 65 million years ago, the central area of Australia was an inland sea. Fine marine sands rich in silica were deposited around the shoreline. The Great Artesian basin formed when the sea receded. Around 30 million years ago, deep weathering caused changes to the sediments. As the water filtered down, it picked up silica from sandstone, and carried the silica rich solution into cracks and voids, caused by natural faults or decomposing fossils. As the water evaporated, it left behind a silica deposit. This cycle repeated over very long periods of time, and eventually opal was formed. Opalised shells, woods and reptilian bones of the Cretaceous period are also found, their remains dissolved by the solution and replaced with opal. The solution is believed to have a rate of deposition of approximately one cm in five million years at a depth of approximately 40 metres.


 This diagram appeared in the Australian Geographic of July-September 1998.
 
Precious opal is one of the most precious gemstones and it shows a variable interplay of internal colours and even though it is a mineraloid, it has an internal structure. At micro scale precious opal is composed of silica spheres some 150 to 300 nm in diameter in a hexagonal or cubic close packed lattice. The coloured lights (called iridescence) that precious opals give off are caused by diffraction of white light from these tiny silica spheres inside the gemstone. A similar phenomenon occurs in iridescent bird wings. It is said that the larger the internal spheres, the larger the range of colours. In some instances one can make an opal more colourful simply by holding it and the heat in your hands will expand the spheres and increase the range of colours.

Depending on the conditions in which the opal formed, it can take on many colours. It ranges from clear, white, gray, red, orange, yellow, green, blue, magenta, rose, pink, olive, brown, black, etc. and if you think the range of colours is extensive; one source mentions over 70 varieties of opal, although not all are recognized as natural opal. This is pretty complex for a mineral that is not really a mineral. Black opal, crystal opal, boulder opal, white opal and fire opal, are just some of the more well-known varieties. Black opal is the most valuable and desired variety and can even fetch a higher price per carat than diamonds. White and precious fire opal can also be quite expensive.

Most precious opal is mined in Australia and is, therefore, also the national gemstone of Australia, which produces 97% of the world’s supply. Other worldwide deposits of precious opal occur in Mexico, USA, Ethiopia, Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Guatemala, Nicaragua, Brazil and Honduras. Very interesting to note is that in late 2008, NASA announced that they had discovered opal-bearing deposits on Mars …………. and no it’s not an April Fool’s joke.

The town of Coober Pedy in South Australia is a major source of precious opal. The world’s largest and most valuable gem opal “Olympic Australis” was found in 1956 at the “Eight Mile” opal field in Coober Pedy. It weighs 17 000 carats (3,45 kg) and is 28 cm long, with an height of 12 cm and width of 11 cm. It was valued at approximately AUD$ 2 500 000 in 2005 and consists of 99% gem opal and the remaining 1% being the soil still attached to it.

The largest opal matrix was also discovered in South Australia weighing 55 000 carats. It was discovered by Stuart Hughes and its dimensions are 30 cm x 20 cm x 4cm and is valued at US$ 1 000 000.

Just some additional useless information on Coober Pedy, with its population of approximately 3500 people, is that it is also a famous tourist destination since 80% of its population lives underground due to the summer temperatures that can exceed 50 degrees. These underground residents have all the modern facilities which you can expect from any normal house. The “Desert Cave Hotel” is also very prestigious accommodation for tourists with the world’s only underground bar, gaming room and art gallery. Although it is a small town, you can experience the incredible ethnic diversity with over 45 different nationalities.

On home turf precious opal has to date not been found in South Africa but common opal in a variety of colours and patterns occurs near Postmasburg and Pella in the Northern Cape, in the Pilansberg and near Soutpansberg.

In the Erongo region in Namibia, hyaline (also a variety of opal) is found mainly as colourless to white/light green globular masses on mineral specimens.
    
Erongo hyaline opal on quartz crystals in normal and shortwave UV light(photo and specimen JW)


Due to their unorganised structure, opals have no crystal shapes to hold them together and they may eventually dry out and crack. Opals are very fragile when exposed to the air as they lose molecules of water and develop tiny fractures. When heated, they can also lose their water content, decompose and may alter into chalcedony or quartz. As a word of caution, opal is apparently soluble in hot salt water. There is some good news, however, since it is insoluble in most acids.

Opals are also relatively soft and for this reason, they need to be set in jewellery in such a way that it affords them the maximum protection against scratching and knocks. Opals must also be cut slowly and with plenty of coolant due to their heat sensitivity. This might seem like a lot of trouble, but is all worth it when set in such a way that their beauty and colour play and flashes captivate the gem lover’s eye.

For gemstone use its natural colour is often enhanced by placing thin layers of opal on a darker underlying stone (doublets), such as basalt. Opal doublets are sometimes also coated with a thin layer or dome shaped clear quartz which not only protects the opal but also acts as a magnifier to emphasize the play of colour and are referred to as triplets.

Common opal is used in the industry as fine abrasives, filtering powders, in insulators and in ceramics. Common opals, also referred to as potch opals, are those stones without the play of colour and they occur worldwide.

The discovery of the ordered sphere structure of opals led to the fabrication of synthetic opal by Pierre Gilson in 1974. This material is distinguishable from natural opal by its regularity under magnification and it also does not fluoresce under UV light, unlike the real McCoy - JDJ
 
References:
Cairncross, Bruce, 2004 – Field Guide to Rocks & Minerals of Southern Africa.
Orbis Publishing Ltd, 1995, - Treasures of the Earth - The Minerals and Gemstones Collection.
Simon and Schuster, 1977 - Simon and Schuster’s Guide to Rocks and Minerals
www.minerals.net
www.mindat.org
www.traveltourimmigration.blogspot.com
 
All specimens and photographs by Johann de Jongh unless otherwise noted.