Volume 20, Number 2, April-June 1998

AN OCCURRENCE OF MICROCRYSTALLINE OPAL
NIPPLE MOUNTAIN, KELOWNA, BRITISH COLUMBIA, CANADA.

Robert R. Coenraads
Opal Mining and Exploration N.L., Sydney
Ross E. Pogson
The Australian Museum, Sydney

ABSTRACT
Colourless and yellowish-brown opal samples from Nipple Mountain in British Columbia (Canada), submitted to Opal Mining and Exploration N.L. for examination, were identified by gemmological testing and x-ray diffraction as microcrystalline opal (opal-CT). Although some of the botryoidal opal would make quite attractive collector's specimens, the material examined did not show the play-of-colour associated with precious opal. Massive pieces were quite heavily fractured.

PORTABLE DARK-FIELDS
G.A.A. Instrument Evaluation Committee Report

B. Bretherton

ABSTRACT
A Gemmological Association of Australia Instrument Evaluation Committee Report on two portable dark-fields manufactured by Australian gemmological equipment maker, Linton Enterprises, of Victoria Point, Queensland.

EMERALD CHEMISTRY FROM DIFFERENT DEPOSITS
An electron microprobe study

Dr. I.I. Moroz,
Institute of Earth Sciences, The Hebrew University of Jerusalem, Israel
I.Z. Eliezri
Colgem Ltd, 52 Bezalel St, 52521 Ramat Gan, Israel

ABSTRACT
Emeralds from eleven gem mining regions, and three hydrothermally grown synthetic emeralds, were characterised chemically by electron microprobe analysis. Emerald occurrences in Australia, Brazil, Mozambique, Russia, Tanzania and Zambia, which belong to 'schist-type' deposits, are characterised chemically by high concentrations of MgO (0.7-3.1 wt%), FeO (0.3-1.8 wt%) and Na₂O (0.2-2.8 wt%). In contrast, Colombian and Nigerian emeralds are characterised by low contents of magnesium (0.00-0.7 wt%) and sodium (0.00-0.65 wt%). The synthetic emeralds had the least amount of sodium.

FOREIGN AFFAIRS

FRACTURE HEALING/FILLING OF MÖNG HSU RUBY

Richard W. Hughes ¹,
Olivier Galibert ^2
1 Boulder, Colorado, USA
² Hong Kong, China

ABSTRACT
Value-enhanced glass fracture-filled rubies from Möng Hsu, Burma, pose a problem of disclosure that is yet to be resolved by all parties concerned. The analysis of this problem presented in this paper has the aim of stimulating discussion that may provide an answer to a vexatious problem that the world gem trade must face.

NEW OCCURRENCE OF DEMANTOID IN NAMIBIA

Th. Lind, U. Henn, H. Bank
Idar-Oberstein, Germany

ABSTRACT
The physical and chemical properties of a new occurrence of demantoid garnet in Namibia are described. The density has been measured as 3.81-3.85 gcm-3, and the refractometer reading on standard gemological instruments is negative. Chemical analyses show a pure andradite garnet with very low contents of Cr₂0₃ between 0.02 and 0.13 wt%. VIS-absorption spectra show absorption bands of Fe³⁺ and Cr³⁺. With the microscope, distinct growth zoning is visible in addition to yet unidentified ore-like inclusions. Chrysotile (serpentine asbestos) inclusions have not been found.

RARE GEM MINERALS FROM BRAZIL
PART 1: EUCLASE AND PHENAKITE

Dr M.L.S.C. Chaves ¹,
Dr. J. Karfunkel DGemG ¹,
Dr. D. B. Hoover FGA ^2
1 Department of Geology and Natural History Museum,
Federal University of Minas Gerais, Belo Horizonte, Brazil
² Hoover Associates, Springfield, USA

ABSTRACT
The principal deposits of the two rare beryllium-bearing gem minerals euclase and phenakite are located in Brazil. Euclase was first discovered over 200 years ago near the city of Ouro Preto, Minas Gerais State. Today, eight deposits - some of which have only been discovered recently - are known in Brazil. These are described in this paper. The optical and physical properties of euclase from each deposit are similar; but associated minerals and crystal habits differ between deposits.

Phenakite was discovered 80 years ago in Brazil, at Rio Piracicaba, Minas Gerais State. This deposit has produced the finest gem and specimen material from Brazil. Unfortunately, it is now worked out. Two other Brazilian deposits of phenakite are known - one of which presently produces facetable material. Due to their scarcity cut examples of these minerals are much sought-after by gemmologists and gemstone collectors.