Along with the alternate birthstone tourmaline, opal is the modern birthstone for October and the traditional birthstone for the Zodiac sign of Libra. Opal is considered the birthstone for people born in October or under the sign of Scorpio and Libra. In late 2008, NASA announced that it had discovered opal deposits on Mars. The word opal is adapted from the Roman term opalus, but the origin of this word is a matter of debate. However, most modern references suggest it is adapted from the Sanskrit word Upala. However, the argument for the Sanskrit origin is strong. The term first appears in Roman references around 250 B.C., at a time when the opal was valued above all other gems. The opals were supplied by traders from the Bosporus, who claimed the gems were being supplied from India. Before this the stone was referred to by a variety of names, but these fell from use after 250.
Of superstitious origin, throughout the Middle Ages, opal was considered a lucky stone because it possessed the collective virtues of similarly tinted gemstones. It was also said to confer the power of invisibility when wrapped in a fresh bay leaf and clenched in the hand. Following the publication of Sir Walter Scott’s ‘Anne of Geierstein’ in 1829, the opal acquired a less auspicious reputation after the Baroness of Arnheim wore an opal talisman with supernatural powers. A drop of holy water fell on the talisman and the opal turned into a colorless stone, shortly after which the Baroness died. Within a year of the publishing of Scott’s popular novel in April 1829, the sale of opals in Europe dropped by 50%, and remained low for the next twenty years or so.
Australia produces around 97% of the world’s opal, of which 90% is referred to as ‘light-opal’ or white and crystal opal. White makes up 60% of the opal productions but cannot be found in all of the opal fields. Crystal opal or pure hydrated silica makes up 30% of the opal produced, 8% is black and only 2% is boulder opal. The term opalescence is commonly and erroneously used to describe this unique and beautiful phenomenon, which is correctly termed ‘play of color’. Contrarily, opalescence is correctly applied to the milky, turbid appearance of common or potch opal, which typically doesn’t exhibit play of color.
Besides the gemstone varieties that show a play of color, there are other kinds of common opal such as the milk opal, milky bluish to greenish (which can sometimes be of gemstone quality); resin opal, which is honey-yellow with a resinous luster; wood opal, which is caused by the replacement of the organic material in wood with opal; menilite, which is brown or grey; hyalite, a colorless glass-clear opal sometimes called Muller’s Glass; geyserite, also called siliceous sinter, deposited around hot springs or geysers; and diatomite or diatomaceous earth, the accumulations of diatom shells or tests.
Fire opal, Mexican opal or Girasols, are transparent to translucent opals with warm body colors of yellow, orange, orange-yellow or red. They do not usually show any play of color, although occasionally a stone will exhibit bright green flashes. The most famous source of fire opals is the state of Querétaro in Mexico; these opals are commonly called Mexican fire opals.
Fire opals that do not show play of color are sometimes referred to as jelly opals. Peruvian opal (also called blue opal) is a semi-opaque to opaque blue-green stone found in Peru which is often cut to include the matrix in the more opaque stones. It does not display pleochroism.
The town of Coober Pedy in South Australia is a major source of opal. The world’s largest and most valuable gem opal ‘Olympic Australis’ was found in August 1956 at the ‘Eight Mile’ opal field in Coober Pedy. It weighs 17,000 carats, measures 11 inches long and 4 3/4 inches high with a width of 4 3/4 inches, valued at AUD$2,500,000.
Mintabie Opal Fields located approximately 250 km north west of Coober Pedy has also produced large quantities of Crystal opal as well as the rarer black opal. Over the years it has been sold overseas incorrectly as Coober Pedy Opal. The black opal is said to be some of the best examples found in Australia.
Andamooka in South Australia is also a major producer of matrix opal, crystal opal, and black opal while Lightning Ridge in New South Wales, is the main source of black opal, defined as opal containing a predominantly dark background. Boulder opal consists of concretions and fracture fillings in a dark siliceous ironstone matrix, found sporadically in western Queensland, from Kynuna in the north, to Yowah and Koroit in the south.
The rarest type of Australian opal is ‘pipe’ opal, closely related to boulder opal, which forms in sandstone with some iron-ore content, usually as fossilized tree roots. Its largest quantities are found around Jundah in South West Queensland. Australia also has opalised fossil remains, including dinosaur bones in New South Wales, and marine creatures in South Australia.
Another common claim that the term 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’, the other is ‘other’ as in ‘alias’ and ‘alter’. It is claimed that opalus combined these uses, meaning ‘to see a change in color’. However, historians have noted that the first appearances of opillos do not occur until after the Romans had taken over the Greek states in 180 B.C., and they had previously used the term paederos.
The Virgin Valley opal fields of Humboldt County in northern Nevada produce a wide variety of precious black, crystal, white, fire, and lemon opal while the black fire opal is the official gemstone of Nevada. Most of the precious opal is partial wood replacement. Miocene age opalised teeth, bones, fish, and a snake head have been found. Some of the opal has high water content and may desiccate and crack when dried.
The largest black opal in the Smithsonian Institution comes from the Royal Peacock opal mine in the Virgin Valley. Another source of white base opal or creamy opal in the United States is Spencer, Idaho. A high percentage of the opal found there occurs in thin layers. Other significant deposits of precious opal around the world can be found in the Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Brazil (in Pedro II, Piaui), Honduras, Guatemala, Nicaragua and Ethiopia.