Volume 21, Number 07, July - September 2002
V21 Number 6  

  • Classification of Type 1 Natural Opal

  • Synthesis of All-silica Precious Opal

  • New Australian Imitation Gemstones

  • Treated Orange Sapphires from Thailand

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CLASSIFICATION OF NATURAL OPAL TYPE 1

Josef Schellnegger
 Lightning Ridge Opal Advisory Service, Lightning Ridge, New South Wales

ABSTRACT
This paper details a practical method of assessing and describing the quality (value) of precious opal from the Lightning Ridge District that has been developed in Lightning Ridge by the Opal Advisory Service. The system is based on quantifying the positive factors that determine the value of opal (body tone, brilliance, pattern, thickness of colour bar, and colour), by deducting certain fault factors (cracks and crazing. sand or gypsum in the face, potch in the face, colour not facing, excessive potch backing, and poor cut, shape and polish), and then describing the shape in which the opal has been cut. Use of an associated code allows individual opals to be accurately and reproducibly described.

SOME ASPECTS OF PRECIOUS OPAL SYNTHESIS

S.V. Filin, A.I. Puzynin, V.N. Samoilov
Scientific Center for Applied Research, Dubna, JINR, Russia

ABSTRACT
In this paper the authors describe the developmental research and basic steps involved in their synthesis of all-silica opal at the Center for Applied Research in Dubna, Russia. The structure and essential features of this attractive, relatively non-porous synthetic opal are illustrated, and cut and polished examples of this synthetic opal are illustrated.

NEW IMITATION GEMSTONES FROM AUSTRALIA

G. Brown
ALLGEM Services, Albany Creek, Queensland
T. Linton
Linton Enterprises, Wellington Point, Queensland

ABSTRACT
A range of semitranslucent to opaque imitations of chrysoprase, jadeite, turquoise, and other ornamental materials is being manufactured in Brisbane by Australian Crystallization Technology Pty Ltd. These very effective imitations are produced by controlled devitrification of melts of proprietary composition that are based on finely ground chrysoprase. As a by-product of this process a number of attractive transparent glasses (supercooled melts) of various colour also are being manufactured. Some of these facet grade products are uniquely multicoloured. This paper will present the history of these new products, and general method used in their manufacture, before illustrating the various products, detailing their gemmological properties and features, and offering guidance with respect to their identification.

ORANGE SAPPHIRES OR JUST LEMONS?

Terry Coldham
Sydney, Australia

ABSTRACT
Based on thirty years of active interest and experience in the heat treatment of corundum, the author provides an update on the latest Thai heat treatment process that results in spectacular changes of colour in ruby and sapphire¾particularly with respect to the commercial production of various shades of orange sapphire.