History of Opals

About Opal - History and Introduction

Uses

Opals are used in jewellery and ornaments.

History                                                                                                           Opal artefacts several thousands of years old have been discovered in East Africa. As early as 250 BC the Romans prized opals, thought to have come from mines in Eastern Europe, the ancient world's main source of opals. There are many aboriginal dreamtime stories that feature opal.

Australian opals discovered during the late 1800's found little favour with European markets but their commercial value increased in the 1900's and in 1932 Australia took over as the major producer of opals in the world and remains the largest producer to this day.

In 1915 a group of people were prospecting for gold at the edge of the Great Victoria Desert northwest of Adelaide. Making camp one night, a 14-year-old boy found an opal. This started an 'opal rush' and soon the settlement of the Stuart Range Opal Field was founded, better known now as Coober Pedy and together with Lightning Ridge in NSW, the bulk of the world's opal continues to be produced.

Formation

In Australia, precious opal is found in Cretaceous age sandstones and mudstones. These sedimentary rocks were deeply weathered, and this weathering released silica into the groundwater. Small faults and joints in the rocks formed pathways for movement of the groundwater as it penetrated downwards.

Impermeable barriers between the sandstone and the underlying rocks trapped the silica-carrying groundwater where it slowly hardened into a gel forming opal in veins and lenses.

Opals are frequently layered and if a rare red layer is present it is at the base in the thinnest portion of the vein and indicates that gravity played a part in the arrangement of the silica spheres.  

Australia is the only part of the world where opalised animal and plant fossils have been found. At Lightning Ridge in NSW, small opalised dinosaurs and primitive early mammalian remains, together with shallow marine shellfish and crustaceans have been found. Probably the most famous opalised fossil is Eric the Pliosaur (Cretaceous age marine vertebrate) which was found at Coober Pedy and now forms part of the Australian Museum collection.

Opal is found around the world (Brazil, Mexico, Honduras and the western US) however Australia produces 95% of the world's precious opal and it is our official national gemstone. Opals can be divided into three main subgroups: precious opal, fire opal and common opal (potch).

Opal is famed for its ability to diffract light. The exact cause of opal's unique properties was only recently discovered by Australian scientists in the 1960s after analysis with electron microscopes. It was discovered that small spheres of silica gel caused interference, refraction and diffraction of light, resulting in opal's distinctive play of colour. The varying refractive indices of the spheres and spaces between them dissect the light on its passage through the stone. As light enters the opal, it bends around the tiny particles or 'spheres' of hydrated silica, as well as 'chips' of silicon and oxygen suspended within the stone. Light is comprised of all visible colours and can produce an entire spectrum of colours when it is diffracted.

Precious opal is known for its remarkable ability to diffract light, which results in rainbow-like colours that change with the angle of observation - known as 'play of colour'. Fire opal can sometimes exhibit slight colour play, but it is better known for its vivid body colour. Common opal is usually opaque, rarely translucent, and lacks play of colour. It is often found mixed with other gemstones, such as agate opal or moss opal. Common opal is known to exhibit 'opalescence'. The term 'opalescence' is often mistaken for 'play of colour'. Opalescence should technically only be used to describe the optical effects seen in common opal. Opalescence is caused by the reflection of light and appears as a sheen of light, typically milky-bluish in colour. It is a form of adularescence, whereas 'play of colour' is iridescence caused by light diffraction.

Properties

Opals are multi-coloured and consist of small spheres of silica arranged in a regular pattern, with water between the spheres. The spheres diffract white light, breaking it up into the colours of the spectrum. This process is called 'opalescence'. Larger spheres provide all colours, smaller ones only blues and greens. Opals that have a predominantly red colour are very rare as they only occur where larger silica spheres were deposited.

White opals have delicate, pale colours on a lighter background. Black opals (very rare and valuable) have a dark background and colours ranging from brilliant red through to greens, blues and purples. Boulder opals are cut with the natural host rock, ironstone, on the back.

The Properties of Opal

Chemical Symbol

SiO2nH2O

Name

There is some uncertainty around the origin of the word opal - it may come from the Greek opallos meaning 'to see a change (of colour)' or may be Sanskrit for ‘(precious) stone’.

Ore

Opal ore

Relative density

2.09 g/cm3

Hardness

5.5-6 on Mohs Scale

 

https://www.ga.gov.au/education/classroom-resources/minerals-energy/australian-mineral-facts/opal