Volume 22, Number 2, April - June 2004

Volume 22, Number 2  
  • The origin of precious opal
  • Rare Australian Gemstones - Crocoite
  • Spinel from Kayah State, Myanmar
  • Inclusions in Vietnamese Quy Chau ruby and their origan
  • Quality enhancement of Vietnamese ruby by heat treatments

(Follow this link for abstracts of past issues)

THE ORIGIN OF PRECIOUS OPAL 

Byron Deveson 
Canberra

ABSTRACT
A new model for the formation of precious and potch opal is proposed. The essential components of this model are mound spring waters of appropriate chemistry: a mechanism whereby the physico-chemical properties of this water are changed so that suitable silica spheres, and then linear chains of these spheres, are formed; and suitable voids that are lined with clay that can act as a semi-permeable membranes to concentrate and purify the silica sol by ultra filtration and dialysis.

RARE AUSTRALIAN GEMSTONES - CROCOITE 

S.P. Sorrell 
Allens Rivulet, Tasmania

ABSTRACT
Crocoite, a brilliant orange to red lead chromate, is rare in nature and is rarely faceted into attractive gemstones. This paper describes the occurrence of crocoite in Tasmania in particular, and its gemmological properties. 

SPINEL FROM KAYAH STATE, MYANMAR

Professor U Tin Hlaing*
Geology Department, Taunggyi University, Myanmar
* Presently Assistant Rector of a new university at 
Pinglong that is located 3 hours drive to the east of Taunggyi

ABSTRACT
 During 2001, gem quality spinels, of predominantly pink to red colour, were recovered from an alluvial deposit in Myanmar's Kayah State. This deposit is located between the Pawn Chaung and Salween Rivers some 40 km south of the township of Hsa-taw. The spinels, some of which 
have been facetted and sold in Taunggyi, the capital of nearby Shan State, are likely derived by weathering, erosion, and transport of marbles associated with the Pawn Chaung series of rocks.

INCLUSIONS IN VIETNAMESE QUY CHAU RUBY AND THEIR ORIGIN

Pham Van Long1, Hoang Quang Vinh2, Nguyen Xuan Nghia3
1Centre for Gem and Gold Research and Identification, 91 Dinh Tien Hoang, Hanoi, Vietnam
2Institute of Geological Sciences, NCNST, Nghia Do, Cau Giay, Hanoi, Vietnam
3Institute of Materials Science, NCNST, Nghia Do, Cau Giay, Hanoi, Vietnam

ABSTRACT
 In this paper the authors present a study of inclusions in ruby from Quy Chau, Vietnam. The characteristics of these inclusions were examined by optical microscopy. Compositions of inclusions were analysed by Raman scattering and SEM (Scanning Electron Microprobe) methods. An assemblage of anatase, andalusite, anorthite, apatite, biotite, boemite, brookite, calcite, corundum, diaspore, dolomite, graphite, margarite, muscovite, phlogopite, pyrite. rutile, zircon and zoisite inclusions were found in this ruby, Compositions of gas-liquid phases in multi-phase inclusions and negative crystals in this ruby have been described, and the conditions of formation of the host ruby also have been discussed.

QUALITY ENHANCEMENT OF VIETNAMESE RUBY BY HEAT TREATMENTS

P. Wintotai1, P. Limsuwan2,
I.M. Tang3, S. Limsuwan2
1Department of Chemistry, Mahidol University, Bangkok, Thailand
2Department of Physics, King Mongkut's Universty of Technology, Bangkok, Thailand
3Department of Physics, Mahidol University, Bangkok, Thailand

ABSTRACT
 A suitable heating condition, specifically designed for a particular ruby, must be used to enhance both its quality and value. Heating Vietnamese rubies in an oxygen atmosphere improved their color and clarity and thus increased their value. Rubv is coloured bv trace amounts of Cr3+, Fe3+, Fe2+ and Ti4+ ions. The Fe3+ yields a pale yellow colour, while the charge transfer mechanism between Fe2+ and Ti4+ gives ruby an undesirable bluish colour. This undesirable colour is reduced by heating the ruby in oxygen so that most of its Fe2+ can be converted into Fe3+ ions. This gives a more intense red from the Cr3+ present in the ruby. Based on X-ray Diffraction (XRD), the c/a ratio of the hexagonal structure of ruby was smallest after heat treatment at 1300 °C. Furthermore, the number of Fe2+ ions that were converted to Fe3+ - as detected by the electron spin resonance spectrometer (ESR) - was found to increase with treating temperature. Vietnamese ruby was converted into a most intense red after optimal heating at 1500 ° C in oxygen for 12 hours. This color enhancement reflects both a decrease in c/a ratio of ruby 's unit cell and an increase in Fe3+ ions in the ruby.