With its huge area and range of geological environments, Australia produces a number of different gemstones. Diamond, opal and sapphire are of major economic importance. Chrysoprase, jade and emerald are mined. Garnet, zircon, quartz, topaz and other gem minerals provide an incentive to fossickers.
Diamond consists of only one element - carbon, although it commonly contains traces of nitrogen (up to 0.23wt %). It crystallises in the cubic system. The octahedron is a common form but many mixed forms occur, as a result of formation and growth conditions. Twinned crystals (macles) are common.
Diamond has a number of outstanding properties that make it a superior gemstone:
Diamond's specific gravity is 3.52, making it one of the heavy minerals in alluvial concentrates.
Diamonds increase in value with 'whiteness', clarity and size. However, fancy colours, such as blue, green, yellow, orange, pink and red are extremely rare and therefore extremely valuable.
There have been many diamond occurrences recorded in Australia but the largest and most economic deposit to date is the Argyle Mine, in the Kimberley region of Western Australia, which produces a huge quantity of stones each year for the international market. Most are used in industry, with approximately 5% being suitable for gemstones. The Argyle mine is unique for the rare pink to red diamonds that form a minute but dependable percentage of its production.
Numerous diamond occurrences have been reported in New South Wales and some areas have been mined. The most consistently mined occurrence has been Copeton/Bingara, in New England. Diamonds were discovered at Bingara in 1872 by tin prospectors and the area has been worked intermittently over subsequent years. The most productive mine was the Monte Christo mine.
In 1875, diamonds were discovered by tin miners at Copes Creek and in 1883, New South Wales' richest diamond field was discovered at Copeton, south-west of Inverell. Basalt capped leads in the area were worked on and off until 1922 and the most productive mine was probably the Star of the South. Since the 1970s there has been renewed diamond exploration and some test mining in the Copeton/Bingara area.
Stones from these areas are yellow or white, highly resorbed and generally three or more to the carat.
Internal dislocations (called gnaats), causing different directions of hardness, made the stones difficult to cut in the past, but today's technology has eliminated this problem.
Opal is a hardened gel of silica and water. Unlike other minerals it is amorphous and has no crystal structure. The amount of water in the structure can vary considerably, from 3 to 20 weight percent. Most opal is a common mineral, found in many countries of the world and in different geological environments. In order to distinguish such opal from the precious opal used for gemstones, we call it common opal. Common opal has no play of colour, although it can be attractively coloured yellow, brown, green, blue or pink by other materials.
When people speak of opal they are generally referring to precious opal, which exhibits a pattern of iridescent colours that roll over the stone as it is moved. This unique effect is called 'play of colour'.
Precious opal is composed of a network of submicroscopic spheres of aqueous silica, separated by voids. This alternating structure acts as a three-dimensional diffraction grating, splitting white light into its spectral colours.
The colours seen in precious opal are dependent upon the size of its spheres. The smallest spheres diffract light of short wavelength and produce violet; larger spheres can diffract violet, blue, green and other colours. The largest spheres diffract red, the light of longest wavelength, as well as all colours of shorter wavelength. For this reason, opals containing red are the most colourful and usually the most valuable.
Dislocations occur in the regularity of the opal structure, causing colour boundaries. These bounded areas of colour produce the many different 'patterns' of opals and ensure the individuality of each stone.
To enhance their pattern of shifting colours, opals are cut and polished into a cabochon. Some of the more attractive patterns are given names, such as flame, harlequin, mosaic, pinfire, Chinese writing, ribbon, flash etc., to describe their appearance.
Opal can be opaque, translucent or transparent, with or without play of colour. Different types of opal have been given descriptive names, many of them coined on the opal fields. Efforts are continuing to standardise opal terminology so that it can encompass opals everywhere in the world.
Precious opal is called dark or light, according to its body tone (the background to its play of spectral colour). Because dark and black opals provide the strongest contrast to brilliant colour, they are the most rare and valuable. Some opal has a coloured body tone, such as yellow, orange, red or brown.
When opal is composed of irregular silica particles of different shapes and sizes, there is no diffraction effect or play of colour. This type of opal is called common opal.
Common opal is usually milky white, grey or brown but can be coloured yellow, pink, blue or green by traces of other material. Potch is a miners' term for a grey common opal found on the Australian opal fields. It has been estimated that most of Australia's opal is potch, with precious opal making up only a small percentage.
Most of the world's black opal comes from Lightning Ridge, New South Wales. The mines at White Cliffs, New South Wales supply mainly light opal. Both fields have produced fine, opalised fossils, formed when opal filled spaces left by bones, shells and wood.
Opal can also form in cavities in volcanic rocks. It is found in basalt at Tintenbar and around Dubbo, New South Wales, and in thunder eggs within a rhyolite flow at Mullumbimby.
In South Australia, precious opal is mined at Andamooka, Coober Pedy and Mintabie. New mines continue to be established in both states.
Australia provides about 95% of the world's precious opal. Small quantities of precious opal are also found in Mexico, Brazil and the United States.
The size of the silica spheres determines which colours are seen. The smallest sizes refract only ultraviolet radiation. When the diameter of the spheres increases to 138 nm (a nanometre equals 10-9 m) they can diffract visible light of the shortest wavelength to produce violet; as the sphere diameter increases, blue, green and other colours appear; larger spheres (from 241 nm) produce red, as well as all colours of shorter wavelength. For this reason, opals containing red are the most colourful overall.
Small dislocations in the regular stacking of silica spheres create bounded areas of colour that vary with the angle of incidence of light. In this way, colour patterns are created that shift across the stone, giving each opal a unique character.
The structure of opal is an ongoing subject for research. Recent studies have suggested that, in precious opal, the spheres may better be described as a solid silica matrix containing a network of voids.
Common opal has no play of colour. Its spheres are not uniform but cover a wide range of sizes. When the diameter of a sphere exceeds 333 nanometres, diffraction occurs only in the infrared area of the spectrum, resulting in an opal with no play of colour. It is usually milky white, grey or brown but sometimes it is coloured by other materials to attractive shades of yellow, orange, green, blue or even pink. When such opal is translucent and attractively coloured, it is sometimes cut in cabochons or even faceted.
* Watchpoint: Because opal contains water, it may crack if exposed to sudden or extreme changes in temperature and humidity.
Sapphire and ruby are gem varieties of the mineral corundum and form in the trigonal crystal system. They are both aluminium oxide, but the addition of different trace elements gives the two varieties different colours. All colours of gem corundum except red are called Sapphire. Red corundum is called Ruby. Ruby owes its colour to around 1 wt % chromium, while sapphires derive their colour from trace elements such as iron, titanium and nickel.
Sapphire is found in many east coast locations, from north Queensland to north-east Tasmania. However, the largest and most economic deposits are in New England, New South Wales, around Inverell and Glen Innes, and central Queensland, around Anakie and Rubyvale.
The New South Wales sapphire fields produce Australia's finest blue sapphires, as well as a range of other colours.
Ruby is found at various sites in New South Wales, including the Macquarie and Cudgegong Rivers and the Tumbarumba area. It occurs in significant proportions to the accompanying sapphire near Gloucester, in the drainage of the Barrington volcano. This area has been fossicked and mined in the past for gold and rubies, and is currently undergoing further exploration and mining for rubies.
Emerald is a gem variety of the mineral beryl. It is a beryllium aluminium silicate, crystallising in the hexagonal crystal system. Its intense green colour is caused by traces of chromium, or sometimes vanadium.
Emerald has been mined at Poona and west of Menzies in Western Australia and near Emmaville and Torrington in the New England area of New South Wales.
In 1890, The Emerald Proprietary Company began mining north-east of Emmaville, in New England, New South Wales. It produced a large quantity of pale to bright green crystals and continued operating until 1909.
At Fielders Hill, near Torrington, emerald has also been found as strongly defined zones in crystals of colourless beryl. These crystals may be cut perpendicular to the c-axis to produce a standard emerald.
They can also be cut parallel to the c-axis to produce a striped stone.
Topaz is an aluminium fluorine silicate, crystallising in the orthorhombic system. It is usually colourless, but can occur in a number of colours - blue, pink, red, yellow, green, orange or brown.
Topaz crystals, usually colourless to pale blue, are found at a number of sites in Australia, with the best gem material coming from Oban in New South Wales, Mt Surprise in Queensland and Flinders Island in Tasmania.
In New South Wales, the Oban and Mitchell Rivers in New England have yielded much fine topaz, occurring as waterworn crystals, cleavage fragments and pebbles. Many large and clear gemstones have been faceted from this material.
Quartz is the most common mineral and is present in a wide range of rocks. It is silicon dioxide and forms in the trigonal system. It can occur as large crystals or fine-grained material with crystals invisible to the unaided eye. This fine-grained quartz is called 'microcrystalline'. Quartz has a range of colours and varieties.
Crystallised quartz is often colourless, but may be milky white, pink, purple, yellow, orange, brown or black.
Microcrystalline quartz comes in a variety of colours and patterns and is often used as a lapidary material for beads and ornamental items.
Quartz is found all over Australia, but one of the most prolific sources of fine quartz has been Kingsgate, in north east New South Wales. The purity of the quartz from this locality made it important for use as quartz oscillators in radio communications during the Second World War, until synthetic quartz took its place. Large crystals of colourless and smoky quartz, with some citrine, abounded at Kingsgate.
Gem quartz is widely distributed in New England, New South Wales.
Rutilated quartz, containing golden or reddish needles of rutile, is found at Tingha.
Garnet is the name given to a group of complex silicate minerals with a similar chemical formula, physical properties and a common crystal system. Five of these minerals, almandine, pyrope, spessartine, grossular and andradite, are used as gemstones. Garnets form in the cubic system and are common in metamorphic rocks in Australia. Localities for gem quality garnets are more restricted.
Grossular garnet is found in the Harts Range, Northern Territory, both in its pure, colourless form and with the addition of iron to colour it yellow, orange and red. The yellow and orange colours are known as hessonite.
The red pyrope-almandine series of garnets, with its in-between variety, rhodolite garnet, is found at many Australian gem localities. Almandine is the iron-rich end member, while pyrope is magnesium rich. Gem quality stones are found in New England and other localities in New South Wales.
Spessartine is a manganese garnet, coloured from orange to red. Fine crystals occur at Broken Hill, New South Wales.
Zircon is zirconium silicate and forms in the tetragonal system. It ranges from colourless to yellow, orange, brown, pink, red, blue and green, with many subtle shades in between. These colours are caused by traces of rare, radioactive elements in varying degrees of radioactive breakdown.
Zircon is found, sometimes with sapphire, in many alluvial deposits in eastern Australia, where it has weathered from basaltic, volcanic rocks. Large, gemmy crystals are sometimes found. The largest gem zircons in Australia are found at Mud Tank, in the Northern Territory.
A number of sites in New South Wales, especially the sapphire fields of New England, produce attractive zircons.
Nephrite is one of two gem materials, nephrite and jadeite, that are known as jade. They have a somewhat similar colour range - green jade being the most sought after - and a tough, microcrystalline structure. Jade has been used as a carving material throughout history. Only nephrite is found in Australia.
Nephrite is a magnesium iron calcium silicate. Iron is the colouring agent, and a high iron content causes deep green to black colours while a low iron content results in lighter shades, including the so-called 'mutton fat' jade. Nephrite is a variety of the tremolite-actinolite mineral series which, in turn, is part of the amphibole group of minerals. Amphibole minerals have elongated, fibrous crystals and this fibrous, interlocking structure gives nephrite its inherent toughness and enables it to take a high polish.
There is a large, mined deposit of nephrite at Cowell in South Australia and a smaller one in the New England area, New South Wales, around Tamworth. Tamworth nephrite is generally lighter in colour than Cowell material.
Rhodonite is a manganese silicate and forms in the triclinic crystal system. It occurs in both a crystallised and massive, fine-grained form. The massive material is a popular ornamental stone, used for carved objects and beads. It is deep pink to red and opaque, with dark veins of manganese lending some patterning.
Massive rhodonite is found in the Tamworth area of New England. At Broken Hill, New South Wales, transparent, red, triclinic crystals of rhodonite occur, and are usually seen as fine mineral specimens on matrix. Occasionally transparent rhodonite from this area is faceted, resulting in a rare, collectors' gemstone.
A transparent, yellow gem variety of labradorite feldspar occurs in Australia, in the Hogarth Range, New South Wales, and near Springsure, Queensland. Large, clear stones can be cut from this material, but care is needed because of two directions of perfect cleavage. Hardness is 6 - 6.5.
Fluorite is a calcium fluoride that crystallises in the cubic system. It occurs in a wide range of attractive pastel colours. A hardness of 4 and perfect octahedral cleavage, however, make it a difficult mineral to facet and preserve. Attractive fluorites are found at Luina, Tasmania and a number of sites in Australia. In New South Wales an emerald-green fluorite comes from Emmaville, New England.
Copyright © Australian Museum, 2004