By Dr Emrys Chew

Opal has been prized since ancient times for its iridescence, a magical play-of-colour seen as flashes of the colours of the rainbow. Its name stems from the Sanskrit word for precious stone (upala) and the Greek derivative of this (opallios) that has been translated as meaning ‘to see a change in colour’. In the classical world, where properties were linked with colour, the opal was regarded as possessing the powers and virtues of all gemstones since it exhibited every colour. Pliny the Elder described opal as having ‘the living fire of the garnet, the glorious purple of the amethyst, and the sea-green of the emerald glittering together in an incredible mixture of light’. For this reason, the Egyptians and Babylonians held that opal was a gift of the gods, believing that its colours were due to lightning striking the stones as they fell from the heavens. The Romans saw the opal as a sign of loyalty and hope, though they were willing to acquire the stones whatever the cost. According to Pliny, Mark Anthony is said to have gone to extraordinary lengths—and finally committed homicide—in order to obtain a magnificent opal from the Roman senator Nonius, which he then gave to his lover Cleopatra. At one point, Caesar Augustus contemplated selling up to a third of the Roman Empire for a single opal. In certain Judeo-Christian traditions, the many colours of opal represent the many faces of Eve, the first woman. As a symbol of purity and insight, opals have also been associated with divination and meditation, spirituality and prayer for many centuries.

From a scientific perspective, however, the optical properties of opal are directly related to its chemical composition. Opal is one of the few gem minerals that is non-crystalline (or only poorly so). It is a hydrate of silicon dioxide, a form of non-crystalline quartz or amorphous silica gel that falls within one of two categories: precious opal and common opal. The characteristic iridescence of precious opal is caused by the reflection and diffraction of light from minute, uniformly sized and closely packed silica spheres that make up the opal. In general, the names assigned to different varieties of precious opal depend on their basic body colour: milky or pale in the case of white or water opals; yellow, orange or red in fire opals; and brown, grey, dark blue or black in black opals. But the deeply coloured stones with an intense colour play are the most rare and valuable. Occasionally, beautiful and distinct patterns are created by the colour play, giving rise to more fancy names; for instance, those displaying a kaleidoscopic checkerboard pattern are called harlequin opals, while those possessing the blue-green iridescence reminiscent of a peacock’s tail feathers are known as peacock opals. Often, thin slices of opal are cut in such a way as to retain part of the underlying host rock, usually a dark ironstone matrix; these so-called boulder or matrix opals can sometimes produce extraordinarily scenic effects and mimic the more expensive black opal. At the other end of the spectrum is the common opal. It may show a wide range of colours and patterns but displays no iridescence because it lacks the regular fine structure of precious opal. Having said that, opal in its many forms can take on the external shape of other minerals, animals, and plant remains that have dissolved away and left a ‘mould’ in the surrounding rock into which the opal permeates and solidifies. Examples of opals that have replaced organic material in fossil wood, bones and fruits are exhibited in some of the world’s major natural history collections. In nature, opal forms either in veins in igneous rocks or cavities in sedimentary rocks. It is a relatively porous, soft material that rates between 5.5 and 6.5 on Mohs’ scale. As it also contains varying amounts of water—usually between 5-10 per cent—some material may crack or craze if allowed to dry out too rapidly after being mined. In view of its optical properties and chemical composition, opal used in jewellery is usually cut as cabochons and occasionally carved as cameos, though transparent material may be faceted. In modern times, fragile slivers of opal vein are sometimes backed with dark-coloured stone to form opal ‘doublets,’ or given an added protective capping of clear quartz or plastic to form opal ‘triplets’.

The unique optical and chemical properties of opal go some way towards explaining the many myths and superstitions surrounding it. In ancient times, opals were wrapped in bay leaves and carried in an attempt to make the bearer invisible. Sometimes called the ‘stone of thieves,’ tricksters were said to use opal for this purpose, though few details of the procedure have survived. During the Crusades, a wife would give her husband an opal to carry into battle as a protective amulet. In medieval folklore and Renaissance medicine, the opal was regarded as a variety of opthalmius (eye stone) and was used for treating and relieving eye diseases. An opal placed on the belly of a woman in labour was reputed to ease the pain of childbirth. Opal was also known as the ‘Cupid stone’ because it was believed to attract romance and love and enhance a person’s inner radiance. Greek astrologers and seers used it to bring out a person’s natural psychic powers and facilitate one’s access to past lives. In Amerindian and Australian aboriginal culture, dreaming of an opal was thought to predict opportunities to come. Certainly, the Greeks believed that opals gave their owners foresight and the gift of prophecy, while the Romans thought that opals could bring them great fortune. On the other hand, some believe that opals bring bad luck to all who are not born in the month of October. Several reasons may account for this superstition, including the very fragile and unstable nature of the gem itself, or the gem’s name and its association with the eyes. The cracking and crazing of stones, or careless misreading of the name in some quarters, led to the notion that opals attracted the evil eye and were therefore the unluckiest of stones. It appears that this myth was perpetuated by rulers who stood to gain from the commercial monopoly of opals (a Turkish sultan, for instance); literary figures more concerned with romance than reason (Sir Walter Scott’s Anne of Geierstein); and gem merchants worried about the mining of opals in their territory.

Opals occur naturally in a wide variety of locations. Since Roman times, the white precious opal set in much antique jewellery has originated from volcanic rock in Dubnik near Presov, Slovakia (formerly part of Hungary). The Aztecs also mined opal from volcanic sources in Central America, and some fine pieces were sent back to Europe by the Spanish conquistadors in the sixteenth century. Nowadays, deposits around Queretaro in Mexico still yield superb fire and water opals, but other sources of precious opal have been found in Idaho, Nevada and Oregon. Brazil and South Africa are other significant producers. Since the nineteenth century, however, Australia has emerged as the world’s principal source of white and black opal. The discovery of superior quality gem material in sedimentary rocks in Australia from the 1850s led to the decline of European production and increased importation by collectors as influential as Queen Victoria. The rich deposit at Lightning Ridge in northwestern New South Wales, discovered in the early 1900s, still produces magnificent black opals from sandstones. Known for the frequent strikes of lightning that hammer its ridge, the area has yielded some of the most famous opals, including the 4,000-carat ‘Big Ben,’ the spectacular 2,250-carat ‘Light of the World,’ and the 169-carat ‘Black Peacock’ (now on display at the Smithsonian). Other important discoveries include the opal deposits at Coober Pedy (1915) and Andamooka (1930), both located in the harsh deserts of South Australia. Today, Australian output accounts for well over 90 per cent of the world’s total production of precious opal, and the finest Australian black opals rank among the world’s most expensive gemstones, retailing at anything between $10,000 and $50,000 per carat.

Further Reading

  1. Christopher Cavey, Gems and Jewels: Fact and Fable (London, 1992).
  2. Allan W. Eckert, The World of Opals (New York, 1997).
  3. Cally Oldershaw, Christine Woodward and Roger Harding, Gemstones (second edition, London, 2001).