Jade in Australia

Introduction
Jade has been used and appreciated by humans for thousands of years. It was fashioned into ornaments, tools and weapons by Neolithic people and has been used for religious tokens in cultures such as the Aztecs, Mayas and Maoris. The Chinese have revered jade above all other gemstones and, it has a unique place in Chinese history.

Jade artefacts have been found at ancient sites of Neolithic culture in Europe, Asia, and both North and Central America. Some of the oldest sites, such as one in Siberia, contain jades that may extend man’s use of jade back beyond 5000 B.C.

In China appreciation of jade is as old as the first Chinese myths and legends. The stone has been called the ‘green essence of heaven and earth’ and is considered to unite the five cardinal virtues of charity, modesty, courage, justice and wisdom. When powdered and mixed with water, the Chinese believed that jade provided a remedy for all kinds of internal disorders and prevented decomposition of the body if taken just before death.

Jade carving was well established in China by 1500 B.C. Most objects had ceremonial or ritual uses, and jade was seldom used for tools or weapons. Jade was so highly esteemed during the early Chinese dynasties that one warlord is reputed to have exchanged fifteen cities for a dish made from white jade.

In Central .America, jade carving was highly advanced in both the Olmec and Mayan civilisations by about 1000 B.C. When the Spanish conquistadors landed in the early 15th century A.D. they found that the Aztecs prized jade more than gold.

The word jade is derived from the Spanish piedra deyjada, which means ‘stone of the loins’. When translated into Latin, jade became lapis nephriticus (kidney stone), with some input from the Greek word nephros that described the kidney. Both derivations referred to the curative powers thought to be possessed by jade.

In New Zealand, the Maoris have used jade for axeheads since around 1350 A.D. In recent times New Zealand jade has been used for the carving of popular talismans known as hei-tiki.

Presently, the largest known deposit of high-quality (nephrite) jade in the world is located outside of the small town of Cowell, on South Australia’s Eyre Peninsular.

What is Jade?
The term jade is used to describer two distinct minerals - nephrite and jadeite. Both materials may have a similar appearance, but they can be distinguished readily by gemmologists by essential differences in their specific gravity (3.0 v 3.3) and refractive index (1.62 v 1.64). For both types of jade, texture is as important as their mineralogical composition. The material must be tough, compact, and fine-grained otherwise it is unsuitable for gem purposes, even though its mineralogical composition may be correct.

Nephrite
Nephrite (chen yu or true jade), the traditional Chinese jade, is mineralogically a monoclinic amphibole of variable chemistry that ranges between the end members tremolite [Ca₂Mg₅Si₈O₂₂(OH)₂] and ferroactinolite [Ca₂Fe₅Si₈O₂₂(OH)₂]. The magnesium in tremolite is often partially replaced by iron and the two minerals grade optically and chemically one into the other.

The colour of nephrite varies from white, yellow, green and brown to black, depending on the amount of iron substituting for magnesium. Iron-poor tremolite is white, grey or green; whereas iron-rich varieties (actinolite) are darker green, grey-green, and grading though to black. Cowell jade is nephrite jade that has an iron content of up to 7.9 per cent Fe₂0₃.

Although nephrite jade exhibits minor variations in its mineralogy, its important property is its toughness. For this reason it was used for axe heads and knife blades as well as for delicate, durable carvings. Toughness in this context is the opposite of brittleness, and is a quite different property to hardness, which is simply resistance of a material to scratching. Diamond is the hardest mineral, whereas nephrite is the toughest, rivalling artificial zirconium ceramics. The toughness of nephrite is due to its interlocking meshwork of fine fibres or needle-like crystals which are commonly 0.1-5.0 microns in diameter and 20-150 microns long (one micron is one thousandth of a millimetre). These fibres usually are arranged in bundles.

The toughest nephrite has random orientation of its fibre bundles combined with a very fine grain size. The presence of foliation (aligned fibres or fibre bundles), as well as a coarse grain size reduces the toughness of nephrite.

Jadeite
Jadeite (fei-ts’ui or new jade) was named because of its close resemblance to traditional Chinese (nephrite) jade. Jadeite is a pyroxene mineral that has the ideal chemical composition NaAlSi₂0₆, but is a member of the continuous isomorphic substitution series between acmite [NaFeSi₂0₆] and diopside [CaMgSi₂0₆].

Pure jadeite is colourless; but the presence of other elements such as chromium and manganese produce green and pink/mauve/ purple colours. Jadeite has an interlocking microgranular texture that gives this mineral a toughness only second to that of nephrite.

Many other minerals simulate and are sold as jade, particularly in Eastern and Asian countries. The more important of these jade imitations include chrysoprase (Australian jade), serpentine (Pilbara jade), aventurine quartz (Indian jade), bowenite (New Zealand new jade), massive hydrogrossular garnet (Transvaal jade), massive idocrase (California jade), prase, and dyed chalcedony.

Discovery and History
In 1965, Harry Schiller, a local farmer prospecting in the Cowell area of the Eyre Peninsular (South Australia), collected a 3-4 kilogram boulder of dense hard rock near an outcrop of white, dolomitic marble. In early 1966, this specimen was identified as nephrite at Adelaide University. This identification was confirmed subsequently at the South Australian Museum and Australian Mineral Development Laboratories in Adelaide.

During the following years many attempts were made to develop the deposits, but with limited success. The period from 1966 to 1973 saw many disputes and changes in tenure. Involvement of the South Australian State Government commenced in February 1974 when the potential of the area for commercial production was fully evaluated. The 1974 geological appraisal was followed, in 1976, by Government funding of a trial mining program and an evaluation of the colours, texture and carving potential of the nephrite at O'Halloran Hill College of Advanced Education.

Ninety-one (91) separate jade outcrops were identified in 1974, but subsequent mapping has now increased this to well over a hundred. All outcrops of nephrite are located within an area of about 10 km², referred to as the Cowell Jade Province.

After its formation in 1976, Cowell Jade Pty Ltd became the dominant lease holder and marketeer of Cowell jade. In January 1987, Cowell Jade Pty Ltd became a public company and changed its name to Gemstone Corporation of Australia Ltd. After more than a decade of mixed fortune, the Gemstone Corporation entered an agreement with the Malaysian-based Royal Selangor Group to design, produce and market carvings, and items of jewellery, and tiles from Cowell (nephrite) jade.

Geology
The (nephrite) jade bodies at Cowell typically have elongate, lensoid shapes in outcrop. Host rocks for Cowell nephrite are dolomitic marble and banded calc-silicate of the Early to Middle Proterozoic Minbrie Gneiss Complex. These high-grade metamorphic rocks were produced during the first and second deformational events of the Kimban Orogeny, about 1840 and 1780 million years (Ma) ago respectively. Subsequent retrogression occurred about 1700 Ma with later cross-warping and alteration about 1590-1600 Ma. Nephrite only formed within the alteration/retrogression assemblages.

Alteration zones, consisting of tremolite, chlorite, epidote, clinozoisite/zoisite and talc, occur particularly along the margins of, or in close proximity to intrusive chloritised feldspar rock that has intruded dolomitic marble.

Cowell nephrite formed:

1. Within alteration zones as large lenses up to 40 m long by 3 m wide, conformable with lithological layering.
2. Within cross fractures up to a metre wide parallel to the axial plane of late-stage cross-warping. Nephrite in cross fractures is typically high quality, fine grained, massive, and dark green to black - but sometimes it does occur in rarer translucent light green colours.
3. By irregular alteration of coarse grained, brecciated diopside¾ producing schistose nephrite that often has diopside inclusions.

The Cowell geological setting of Middle Proterozoic dolomitic marble host rocks contrasts with other major occurrences of nephrite in New Zealand, Canada and Taiwan; where nephrite lenses occur within or along the faulted margins of serpentinised ultramafic rocks such as peridotites.

Quality and Value
Cowell jade exhibits a variety of colours and textures, but consists predominantly of medium to fine grained material showing, greenish yellow to green hues, grading to black. Three main varieties are marketed:

• Green nephrite
• Black nephrite
• Premium black nephrite

Dark green to black varieties predominate, however the typical apple- and emerald-green colours of some overseas deposits are lacking.

Cowell black jade takes a very high polish by simple lapidary techniques. Very fine grained premium black takes a mirror-like polish, superior to that produced on jade elsewhere. Spectacular rarer varieties, particularly those derived from rind patterns and dendritic inclusions, greatly enhance the appearance of Cowell jade.

Mining and Presentation
Mining of Cowell jade is undertaken periodically - depending on stock on hand and current demand. The country rock on the hanging wall contact of the nephrite lens is drilled, blasted and removed to expose a wall of jade. Then shot holes are drilled and overburden is removed. The nephrite is broken and levered onto the pit floor by a excavator-mounted hydraulic rock breaker. Rough nephrite is then rucked 25 km to Cowell for sawing and grading according its to colour and quality. Some nephrite is exported in the rough; other jade is processed further by cut and polished for jewellery, or presented as polished ornamental slabs.

Jade of carving quality is first trimmed with a diamond saw, carved using diamond tipped tools, and then polished with a diamond paste to achieve its characteristic final lustre.

References:

• Barnes, L.C. et al. (1980) Nephrite jade. In Some semiprecious and ornamental stones of South Australia. pp. 11-39.Department of Mines & Energy and Geological Survey of South Australia.
• Farrand, M.G. (1985) The genesis of jade and host rocks in DDH14 at outcrop 15, Cowell jade province, South Australia. Retp. Bk No. 85/20. Department of Mines & Energy, South Australia.
• South Australian Department of Mines & Energy (1991) Jade in South Australia. Information Sheet 13.