RARE AUSTRALIAN GEMSTONES
Moonstone
A rare Queensland gemstone

Hylda Bracewell,
Brisbane, Queensland

The word feldspar is derived from the German word feld, meaning field, and the word spar, that describes a shiny rock that is easily cleaved. The original German word for feldspar was fieldspar. When this word was introduced into English literature, it became feldspar, as the 'i' was inadvertently dropped from the spelling of the original German word.

Feldspar is the group name applied to several crystalline aluminosilicate minerals that are found in abundance in the earth's crust. The feldspar group of minerals are found, commonly in pegmatites, in many localities throughout the world. However, it was only at the end of the 18th century that serious research into the various species that form the feldspar group of minerals began.Today, around 40 varieties of feldspar have been identified and described in the literature. Gemmologists are concerned, mainly, with those species of feldspar, and their varieties, that share those desirable attributes that provide gemstone status - beauty, durability, and rarity. Well-known gemstone varieties of feldspar include moonstone, sunstone, amazonite, and labradorite.

Map 1. Location of the Moonstone Hill moonstone deposit. 
(Top map courtesy of the Queensland Department of Mines & Energy)

HISTORY
The island of Ceylon (now Sri Lanka) has, from time immemorial, produced the most desirable and enchanting specimens of moonstone. These occur in colours that range from misty white to soft grey, pale orange, and shimmering silver-white.According to tradition, in the Moonstone Temple of Anuradhapura, the steps of the altar are faced with mosaics of gleaming moonstones.The ruins of this temple, which was built about 100 B.C., still can be seen today.

It is the exsolved lamellar structure of moonstone that gives this gemstone its characteristic beautiful blue shimmer (adularescence or schiller) that seems to floatacross the convex surface of the stone, as it is moved 2 . The term adularescence is derived from the name of the variety of orthoclase feldspar, known as adularia. The name adularia was derived 1 from the locality where this feldspar was erroneously first thought to be discovered - the Adula Mountains (Reheinwaldhorn) in Switzerland. Folk law suggests that moonstone was supposed to awaken tender passions if placed beneath the tongues of lovers at full moon. In addition, it was recommended that holding a moonstone in the mouth would refresh one's memory. Moonstone, which some connoisseurs consider to be the most valuable gem feldspar, has for many centuries been a favoured stone for setting into jewellery. In India, today, moonstone is still regarded as a sacred stone. It is, indeed, the shimmery, bluish light that floats across the surface of this gemstone that makes it almost universally the most desired of the gem feldspars.

LOCATION
A search of literature by the author revealed that mention of the occurrence of moonstone in Queensland could only be found in N & R Perry's Prospectors' Guide to Gemstones in Australia 3 that was published in 1982. This reference gave localities for moonstone at Bell, Greenvale, Minerva Creek, Porters Gap and Springsure. Of these undescribed localities, the Springsure deposit has been shown to be the source of facetable yellow labradorite, and not moonstone.

The deposit of moonstone being described in this paper is located at Moonstone Hill, which is found 4 on the eastern side of the Kennedy Developmental Road, 16km north from Hughenden, and approximately 90 km south of The Lynd, which is situated at the junction of the Gregory and Kennedy Developmental Roads (Map 1). When travelling northwards along the Kennedy Development Road, from the Chudleigh Park turnoff, Moonstone Hill is located immediately on the right after crossing the third cattle grid. A walking track leads from the roadside to the hill. This track literally gleams with small - and some not so small - pieces of the moonstone. Newcomers to this fossicking area could be forgiven for thinking many car windscreens had been smashed and strewn everywhere on this track.

GEOLOGY OF THE MOONSTONE HILL DEPOSIT
Stephenson et al's chapter 'Cainozoic volcanism in northeastern Australia' that was published in Henderson & Stephenson's 1980 monograph The Geology and geophysics of Northeastern Australia 5 indicated that Moonstone Hill was located in the Chudleigh Province - a discrete portion of the continuous basaltic area to the west of Charters Towers that was subdivided into the Chudleigh, Sturgeon and Nulla Provinces.

According to these authors, the Chudleigh Province straddles the Great Dividing Range, with flow systems extending down former valleys to the north, east and southwest. The province is characterised by broad, partly dissected lava plains between numerous pyroclastic cones and lava shields.A low plateau and two small volcanic cones, the Bombarri Crater and Barker Crater, lie in the west of the province. Barker Crater is situated west of Black Braes Homestead.

Moonstone Hill (Figs. 1 A&B), which is located 5 km north of the turnoff to Black Braes station, is one of many scoria vents which are surrounded by flat-lying basalt lavas. These lava plains overlie older metamorphic rocks of Precambrian age, Palaeozoic sediments, granitoids and Cretaceous sediments. 

Scoria, a lava with numerous gas bubbles, which is now exposed across the top of Moonstone Hill can be identified (Fig. 2) by its vesicular texture. 

Fig. 1. Moonstone Hill, North Queensland, the source of anorthoclase moonstone, (LHS) viewed from Black Braes station, (RHS) viewed from the Kennedy Development Road.
Fig. 2. Moonstone bearing scoria from Moonstone Hill.		Fig. 3. Scoria at the top of Moonstone Hill that is the source of the best quality gem anorthoclase.
Fig. 4. Characteristic blue schiller displayed by Moonstone Hill anorthoclase feldspar.		Fig. 5. Fossicking area for moonstone at a small roadside 'quarry' located near the turnoff to Black Braes station.

FOSSICKING FOR MOONSTONE
While Moonstone Hill moonstone can be found in the topsoil along the sides of this small hill, experienced fossickers usually prefer to walk to the top of the hill to dig (Fig. 3). Digging and dry-sieving the top layer of soil will produce an abundance of feldspar fragments in sieves. Usually, as some dirt remains adhering to the fragments of the feldspar, fossickers are wise to take care in choosing their final selection. Good specimens of moonstone in the scoria matrix are also plentiful; however these may have to be selectively removed from the larger rocks. (Also see figure 2).

Colourless, flawless faceting grade feldspar can be found at Moonstone Hill, as can 'textbook' moonstone that displays a sheen of typical bluish adularescence or schiller (Fig. 4).

It should also be mentioned that fossicking at the Chudleigh Park peridot field that lies to the south also produces some specimens of moonstone that occurs together with the peridot and (occasionally) some sapphire. Fossickers, however, should visit Moonstone Hill if they are seeking moonstone of quality as well as quantity. 

Acting on advice freely offered by a road worker, the author also briefly visited a small area near the turnoff to Blackbraes station, which had most likely been an exploration area for suitable road fill. Here, again, moonstone in all its various qualities was found. (See figure 5)

The author visited Moonstone Hill during 1982, and again during1986. Since that time, Moonstone Hill, which is surrounded by the Blackbraes National Park, has become a small multiple-use Resources Reserve.This Reserve was gazetted in 1998 under the Nature Conservation Act 1992, under the joint trusteeship of the Director-General, Department of Environment and Heritage, and the Director-General, Department of Mines and Energy.

Provision for fossicking is contained in the draft Management Plan for this Reserve. In this draft plan the trustees have given a 'general permission'under the Fossicking Act 1994 for fossicking and camping, under specified conditions. 

The Queensland Department of Mines and Energy Information Sheet 'Moonstone Hill feldspar locality' 5 summarises the access, fossicking and camping regulations that apply for this site.

Fig. 6. Characteristic inclusions in Moonstone Hill moonstone. (A) Cleavages intersected at almost 90 ° (x 20) (B) Iron oxide stained surface-reaching cleavages (x 35) (C) Decorative 'puddles' located within 'partly healed' cleavages: viewed with transmitted light (Trans), and dark-field illumination (DF). (x40) (D) 'Centipede' type patterns of intersecting cleavages. (x15)		Fig. 7. Fine albite-pericline 'tartan' twinning displayed by Moonstone Hill anorthoclase feldspar when this is examined between crossed polars. (x40).
Fig. 8. Faceted Moonstone Hill anorthoclase, together with the rough from which it was faceted.		Fig. 9. Cabochoned Moonstone Hill anorthoclase, displaying characteristic bluish schiller (RHS), and no schiller (LHS).

GEMMOLOGY
The feldspar from Moonstone Hill has been identified as anorthoclase, by Brown 6 , and determined it to be an anorthoclase-crypto-perthite antiperthite.

The adularescent moonstone from this deposit displayed the following gemological properties: Inclusions characterising this moonstone included (Fig. 6A-D): - Cleavages that intersected at almost 90 ° - Iron oxide stained surface-reaching cleavages - Decorative 'puddles' located within 'partly healed' cleavages. - 'Centipede' type patterns of intersecting cleavages.

Mineralogically, Moonstone Hill moonstone is an anorthoclase cryptoperthite antiperthite that does display very fine albite-pericline 'tartan' twinning (Fig. 7) when examined between crossed polars.

FASHIONING OF MOONSTONE HILL MOONSTONE
Flawless, colourless fragments of Moonstone Hill feldspar can be faceted (Fig. 8). These gemstones are rapidly becoming collector's items. Feldspar displaying the typical bluish schiller of moonstone should be cut en cabochon. This cut will best display the schiller (Fig. 9) that occurs occasionally in this feldspar. Preferably, these moonstones should be cut so that they have a high convex polished surface through which the gemstone's adularescence will be displayed. Moonstone Hill feldspar also may be manufactured into attractive beads.

CONCLUSION
The gem quality feldspar from Moonstone Hill is indeed a rare gemstone from the northern state of Queensland in Australia. Clean fragments of this adularescent anorthoclase-crypto-perthite antiperthite can be faceted and cabochoned to yield desirable 'collector's stones'. Thanks to the Department of Mines and Energy in Brisbane, this deposit is accessible to all visitors to the area. Despite the remoteness of this deposit, which isolates visitors from large towns and associated conveniences, the lack of drinking water-which must be carried there, and the sheer empty wide-open spaces that are characteristic of this area of Queensland, Moonstone Hill attracts gem seekers from around the world.

With little effort Moonstone Hill will yield its buried treasure ready to be fashioned into a glorious gemstone of great beauty.