Chrysoprase

Chrysoprase is the most valuable variety of microcrystalline quartz (chalcedony). It consists almost entirely of silicon dioxide.

It is fashioned principally into cabochons, beads, bangles, or carved to yield jewellery and other ornamental objects. It is easily worked and takes a fine polish.

Colour and transparency

Due to traces of nickel, chrysoprase is green in colour, ranging from pale green, through apple green to a deep rich green. Research by Vasconcelos and Singh (1996) has revealed that the nickel occurs as platelets of the Ni-talc mineral willemseite.

Chrysoprase is usually translucent, but may be opaque in poorer quality material. A small proportion of stone is semi-transparent to transparent and, if of good colour, is highly prized and valued.

The highest quality material is a rich apple green of even colour, without flaws, fractures, inclusions, cavities or other imperfections.

Gem quality chrysoprase

The identifying properties of gem quality chrysoprase are:

Colour: pale green, yellowish green, apple green, deep green
Diaphaneity: translucent to opaque
Lustre: vitreous to resinous
Hardness (Mohs scale): 6-7
Specific gravity: 2.58-2.64
Refractive index: 1.53-1.54
Birefringence: form birefringence 0.004
Chelsea filter reaction: green
Ultra-violet fluorescence: none

Occurrences

Chrysoprase occurs within the deep weathering profile resulting from lateritisation of nickeliferous serpentinites or other ultrabasic rocks in the Marlborough district, where Neoproterozoic-Early Palaeozoic serpentinised ultrabasics have undergone later lateritic enrichment.

Serpentinised ultrabasics occur in three main areas of eastern Queensland: the Marlborough district, the Widgee-Kilkivan district, and the region between Greenvale and Mount Garnet.

Weathering and oxidation of silicate minerals, such as serpentine, during lateritisation generates iron oxides and releases silica and nickel, which are transported down through the developing laterite profile to be re-deposited in veins and nodules (possibly replacing magnesite) where chemical conditions provide favourable sites.

At Marlborough, the chrysoprase occurs throughout the weathering profile but it is particularly abundant in the magnesite-rich saprolite zone underlying an iron-rich silica cap.

Production

Chrysoprase of high quality has been produced commercially since the early 1960s from the Marlborough district of Central Queensland, 90 km north-west of Rockhampton. The main mine at Marlborough is owned and operated by Gumigil Pty Ltd, a Hong Kong-based company. Its entire production is sent to Hong Kong for processing and sale.

A second smaller mine on adjacent leases has also produced considerable quantities of stone in the past and it has changed ownership a number of times. The mining leases are now held by Marlborough Nickel Pty Ltd but the previous holder Candala Pty Ltd (trading as Candala Chrysoprase) is contracted to retain access to chrysoprase deposits on the leases.

In South Australia, material mainly of carving grade has been won from the Mount Davies (Pipalyatjara) deposits. Most of this material has probably been directed to the US and Korean markets.

Production in Western Australia has come mainly from the Yerilla mine operated by Chrysoprase Mines of Australia (Bellmount Holdings Pty Ltd). Small quantities have also been produced intermittently from deposits at Wingelina and elsewhere

Mining process

Mine workings consist of large open-cuts. Heavy earth-moving machinery is used to excavate benches within these. The barren iron-rich cap of the deep weathering profile, which may be up to 40 m thick, is removed by bulldozer.

The underlying saprolite (soft, earthy, clay-rich) zone, in which the chrysoprase veins and nodules occur, is progressively stripped and the freshly exposed floor is checked for signs of chrysoprase after each bulldozer pass.

When chrysoprase is exposed, the veins are removed by backhoe or excavator, and the chrysoprase is collected by hand. It is later cleaned using a high-pressure water spray; separated by wet trommelling; sorted for quality; and finally packed in drums for dispatch to market.

Outlook

The Queensland deposit of chrysoprase at Marlborough is by far the largest in Australia, and is generally of superior quality. Careful control over production levels has been maintained to avoid flooding markets and adversely affecting prices and demand.

Production from this deposit has been a significant source of supply for world markets, with values averaging above $1million per year from 1990-91 to 1999-00. However, the production value fell by around 70 per cent from 2000-01 and there was no production recorded in 2002-03. Stone from existing stockpiles continued to supplement supplies. There was only limited production in 2003-04 as the anticipated increase in demand by the Chinese market did not materialise. Similar reduced production levels have continued through 2004-07.

At current rates, production from existing resources in Queensland is likely to last for at least 20, and possibly up to 40 years.

Further production from deposits elsewhere in Australia is likely.

References

Vasconcelos, P & Singh, B 1996, 'The crystal chemistry and genesis of chrysoprase,' in Mesozoic Geology of the Eastern Australian Plate Conference, Geological Society of Australia Inc., Extended Abstracts No.43, 545.

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Last Updated 06 May 2008