Australian Gem Gallery - A Rainbow of Gems

Emerald - The Green Beryl

Australia's Emerald Deposits - Grahame Brown

Rough and cut beryl and emeralds from the New England area of New South Wales
Rough and cut emeralds from the Curlew Mine, Western Australia		Beryl crystals from Emmaville, New South Wales
Uncut emerald in matrix from Menzies, Western Australia		Largest cut emerald - 'Big Mumma'  from Menzies, Western Australia  15mm x 9mm

Australia is not a significant producer of gem quality emerald; but it does have some interesting emerald deposits. Over the last century four deposits have been commercially mined for emerald: Emmaville and Torrington in New South Wales, and Poona and Menzies in Western Australia.

New South Wales
Australia first became an emerald producer when D.A. Porter, an Inspector of Public School Buildings, discovered green crystals while prospecting an abandoned tin mining lease known as Cleary’s Lode. This prospect was located in the New England District of north eastern New South Wales — some 9 km north-north-east of the small alluvial tin mining town of Vegetable Creek (now Emmaville). Porter submitted a parcel of these crystals to T.W.E. David, then a Geological Surveyor at the NSW Department of Mines & Agriculture, who reported that "they are beryls of a colour sufficiently emerald green to entitle them to be termed emeralds".

Emmaville emeralds have an interesting history, for they appear to have a pegmatitic origin in which emeralds occur in ‘bunches’ in a solid quartz-topaz-feldspar-mica pegmatite that intrudes surrounding Permian sediments¹. Hand mining of this quartz-topaz-feldspar-mica pegmatitic offshoot of the Moule Granite commenced almost immediately, and over a 20 year period of intermittent production the Emerald Mine produced at least 53,225 ct of mostly pale green emerald of rather mediocre quality. The emeralds were recovered by hand mining a 100 m deep vertical shaft and three horizontal drives through solid granite. Although emeralds recovered from the Emerald Mine were generally small in size, and difficult to remove from their very hard enclosing matrix, they did have an attractive light emerald-green colour due to their low content of Fe³⁺ (0.13-0.25 wt% FeO),V³⁺ (0.08-0.13 wt% V₂O₃), and Cr³⁺ (0.02-0.15 wt% Cr₂O₃). These emeralds also had very low contents of Mg²⁺ (0.03-0.06 wt% MgO) and Na⁺ (0.06-0.08 wt% Na₂O)².

Emmaville emerald has the following gemmological properties¹:

During the early 1990s, a small deposit of emeralds, of quite surprising quality, was discovered in a well decomposed pegmatite that was located under an unsealed road near Torrington, a former tin mining village which is located about 20 km to the east of Emmaville’s historic Emerald Mine³. Mining operations were conducted under great secrecy, and the discovery is now considered to be exhausted. Some emeralds from this find have had asking prices of over $A1,000 per carat.

The emerald crystals from this deposit are strongly colour zoned parallel to the basal pinacoid (0001), and to a lesser extent parallel to hexagonal prism (1010), first-order hexagonal dipyramid (1012), and second-order hexagonal dipyramid (1122) faces⁴. Combinations of this zoning lead to the frequently observed presence of distinct green coloured ‘phantoms’ within crystals. Indeed colour zoning is so strongly developed that these emeralds consist of alternating layers of emerald and colourless beryl. Chemical analyses⁴ have revealed that the dark green emerald sectors had an average content of 0.13 wt% Cr₂O₃, while the colourless sectors contained no Cr³⁺. The presence of up to 0.16 wt% V₂O₃ intensified the emerald green colour of these beryls, while iron (0.20 wt% FeO average) had little influence on their colour.

Gemmological properties determined for these emeralds are³:

Western Australia
Most attempts at economically mining emerald in Australia have been based in Western Australia. In this state, belts of Archean chromium and vanadium-bearing ultramafic greenstones, within the Yilgarn and Pilbara Cratons, have been intruded by beryl-bearing pegmatites. It is this interaction that has generated Western Australia's widespread, though unfortunately to date low quality metamorphically generated ‘black-wall’ schist-type emerald deposits⁵.

Almost all Western Australian commercial emerald production has come from the Poona district, 500 km NE of Perth. Here emeralds were first discovered by the prospector A.P. Ryan in 1912. At Poona green beryl and emerald occur in both the intruding quartz-beryl pegmatite and biotite-phlogopite schists that border the intruded greenstones. At Poona emeralds occur in association with beryl, quartz, albite, oligoclase, topaz, tourmaline, fluorite, biotite, phlogopite, lepidolite, zinnwaldite, margarite, talc, tremolite, muscovite, scheelite, chromite, apatite, manganotantalite, and monazite.

The principal mine in the region, the Aga Khan Deep mine, has a colourful history. It has been intermittently worked both as an open cut and more recently (1977-1981) as an underground operation. In over seventy years the Aga Khan mine has yielded an estimated 10 kg of mostly low value emerald. Inevitably difficulties associated with economic mining and recovery of mostly low grade emeralds from their tenacious enclosing mica schist once again forced the temporary closure of the Aga Khan mine. Many small-sized open cut mines are located to the east of the Aga Khan Deep Mine. These include the Quartz blow, Mid section, Solomon, Reward, and Lee’s trench open cuts.Other emerald mining areas around the Poona District include the Emerald Pool mine, 16 km south-west of Poona, and the Poona East Emerald mine about 10 km east of Poona.

Poona emerald, which has been shown² to contain mean concentrations of 0.04 wt% V₂O₃, 0.24 % Cr₂O₃ and FeO, 0.33% MnO and 0.38% Na₂O, has the following gemmological properties⁶:

Little has been published about other deposits of emerald that are located throughout Western Australia’s Greenstone Belt. For example, other deposits of emerald have been recorded⁹ at:

• Noongal, about 130 km south-west of Poona.
• The Curlew mine on White Quartz Hill some 19 km north-west of Hillside Station.
• Warda Warra and Melville on the Yingarn Craton, or Wodinga, Pilgangooraand McPhees Patch on the Pilbara Craton.

Conclusion
Although little gem quality emerald has been mined from Australia, potential for increased production does exist once technology for safely and economically removing emerald from its tightly-enclosing mica schist has been introduced into the emerald mines of Western Australia.

References:

1. Brown, G. (1984) Australia’s first emeralds. Journal of Gemmology.19, 320-335.
2. Schwarz, D. (1991) Australian emeralds. Australian Gemmologist.17, 488-497, 501.
3. Pearson, G. (1992) Torrington emerald. Australian Gemmologist. 18, 47-49.
4. Schmetzer, K. (1994) Torrington emerald update. Australian Gemmologist. 18, 318-319.
5. Grundman, G. & Morteani, G. (1995) A new occurrence of emerald, alexandrite, ruby and sapphire in topaz-bearing phlogopite rock in Poona, Cur District, Western Australia. Z. Dt. Gemmol. Ges.44 (2/3), 11-31.
6. Graindorge, J.M. (1974) A gemmological study of emerald from Poona, Western Australia. Australian Gemmologist.12,75-80.
7. Garstone, J.D. (1981) The geological setting and origin of emeralds from Menzies, Western Australia. Journal of the Royal Society of Western Australia. 64, 53-64.
8. Whitfield, G.B. (1975) Emerald occurrence near Menzies, Western Australia. Australian Gemmologist.12, 150-152.
9. Geological Survey of Western Australia (1994) Gemstones in Western Australia. GSWA: Perth

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