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Eurojade - Le Spécialiste du Jade - The Jade Specialist

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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 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 diopside 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.

@ J. Townsend, Primaries Industries & Resources, South Australia
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