Feature: Gems of Eastern Australia

Fancy coloured gem corundum suite, Swanbrook field, New England region, northern NSW. Photo: Gayle Webb, Geosciences, Australian Museum, Sydney.
Tuesday, 29 October, 2013

Sapphires and rubies have fascinated mankind for thousands of years due to their beauty; Dr Ian Graham explains their colour, their formation and where they are found.

Sapphire and ruby are both gem varieties of the mineral corundum, which is the trigonal crystal form of aluminium oxide (Al2O3). Pure corundum is white. The blue, yellow and green suite of colours of sapphires is due to small amounts of chromophores, or colouring agents:  Fe2+, Mg2+, Fe3+ and Ti4+.

The pink to red colour of ruby is due to the chromophore Cr3+. Other colours such as mauve and purple may be partly due to trace V3+.

Important properties of corundum are its hardness (9 on Mohs scale), relatively high melting point of 2000 to 2500 degrees, and insolubility in acids.

All precious gems are measured in carat weights, with one carat being 0.2 grams. The most valuable gem corundums are blood-red rubies - known as pigeon-blood rubies - and padparadscha sapphires, which are pink-orange sapphires.

Gem corundums are widely distributed throughout eastern Australia, stretching from Tasmania in the south up to far north Queensland. All are associated with intraplate Cenozoic basalt fields, particularly alkaline basalts that have formed within a tectonic plate in the last 65 million years within these fields.

The main fields include Weldborough in Tasmania; Myrniong in Victoria; Tumbarumba, Barrington Tops and New England fields in NSW; and the Anakie and Lava Plains fields in Queensland. The richest are those of the Kings Plains district near Inverell in north-eastern NSW and the Anakie field of central Queensland.

Importantly, new gem corundum fields are still being found in eastern Australia. Due to their erratic distribution, most gem corundum deposits in eastern Australia are mined by either individuals or small groups of prospectors.

Gem corundums are essentially foreign crystals, or xenocrysts, within the Cenozoic alkaline basalts, having formed from other processes.

In eastern Australia, most deposits of sapphire and ruby occur either as present-day or ancient placer deposits, formed when crystals which have been released from their host rocks by physical and chemical weathering are deposited by gravity within rivers and streams.

The gem corundums are associated with zircons (zirconium silicate) and spinel gemstones.

We know about the geological environments of corundum formation through a number of lines of evidence. They include surface features, such as corrosion pits, and primary mineral inclusions trapped within the crystals, such as rutile, nepheline and zircon.

Other clues come from fluids and melts trapped within the crystals; the isotopic signature of oxygen; trace element abundances, particularly of Cr, Fe, Ti, Ga, and Mg; and the ratios of the corundums  - in particular, Fe versus Cr/Mg, Fe/Mg  versus  Ga/Mg and V-Cr-Fe.

We can directly determine the age of crystallisation of the corundums by using the U-Pb method on zircon inclusions and associated zircon xenocrysts. Additionally, we can determine the temperature and pressure conditions of corundum formation from the chemistry of co-existing mineral inclusion pairs.

Our knowledge on the origin of gem corundums has increased significantly over the last 10 years due to advances in analytical chemistry. In particular, advances in mass spectrometry have enabled us to analyse and obtain radiometric age dates from inclusions within the corundums. It has also enabled us to accurately determine in situ oxygen isotopic compositions, which constrain the source of oxygen within the corundum, thus informing us about the geological environment.

Our current theory for the formation of gem corundums is that the sapphires crystallised from alkaline syenites - igneous rocks low in quartz - within the upper mantle and lower crust regions. It is thought that rubies crystallised from metamorphosed silica-poor igneous mafic rocks – rocks rich in magnesium and iron.

Later, basaltic magmas from deeper in the mantle passed through and fragmented these rocks, entrailing the gem corundums and associated zircons and spinels within them.

Most of the gem corundums were brought to the surface during minor pyroclastic events both early and late within the eruptive history of the basaltic fields. Thus, sapphires and rubies are not only aesthetic gems, they also inform us about geological conditions deep within the earth that we cannot directly access.

This article first appeared in newsletter of the Science Teachers' Association of NSW.

UNSW Science Dean, Professor Merlin Crossley is patron of the association.