Pearls from the lion’s paw scallop

Kenneth Scarratt and Henry Hänni. pp 193-203

Pearls from the Lion’s Paw scallops Nodipecten (Lyropecten) nodosus L. 1758 and Nodipecten (Lyropecten) subnodosus Sowerby 1835 are rare and have only recently come to the notice of the gemmological community. They are non-nacreous but differ in surface appearance and composition to other non-nacreous pearls such as the Conch and Melo varieties. The surface appearance is comprised of a patchwork of cells with each cell being formed from three sub-cells. The orientation of these sub-cells and the low magnification fibrous appearance of structures within them give the scallop pearl a peculiar surface sheen. SG and Raman data indicate that the perfectly round 5.91 ct scallop pearl described is composed in the main of calcite rather that being dominated by aragonite as is the case for the Melo and Conch pearls. In addition the chemistry, infrared, and UV/visible spectra for the 5.91 ct scallop pearl are described. These data are compared with similar data from lion’s paw shells and detailed submicroscopic structures are described for the shells.

A treatment study of Brazilian garnets

Sigrid G. Eeckhout, Antonio C.S. Sabioni and Ana Claudia M. Ferreira, pp 205-214

Over the past decade, there has been noticeable growth in interest in coloured stones worldwide, which had led to an increase in gem exploration, production and marketing. Since garnet displays a very large variety of colours, it deserves further attention. Although reports on enhanced gemstones are widespread in the gemmological literature, very few studies have been performed on the enhancement of garnets. We report the first systematic, scientific treatment study on Brazilian garnets from known geological localities, including thermal and diffusion treatment. Iron-containing species become opaque and produce a ‘silvery skin’. Light yellow grossular turns to orange similar to that of Imperial topaz. Other garnet varieties have stable colours, confirming the absence of colour centres. A preliminary diffusion treatment of some rough grossular has produced attractive green and orange stones. Since orange gemstones are becoming increasingly popular and since the diffusion-treated green grossulars resemble some emeralds in colour, they may be of economic importance in the future if quantities are confirmed to justify commercial mining.

The nature of channel constituents in hydrothermal synthetic emerald

Rudolf I. Mashkovtsev and Sergey Z. Smirnov, pp 215-227

Hydrothermal synthetic emerald manufactured by Biron, Regency and Tairus have been distinguished from natural and synthetic emeralds using chemical analysis and infrared (IR) spectroscopy. Polarized IR spectra have been obtained from plates. Hydrothermal synthetic emeralds possess infrared absorption features not present in natural emeralds, and these features have been investigated. We infer that the system of five narrow bands in the 3000-2600 cm^-1 region is related to the dimer formation of HCl molecules, which have been incorporated in the channels parallel to the c-axis during growth of Biron and Regency emeralds. In Regency emeralds, a double band between 3000 and 2500 cm^-1 are attributed to hydrogen-bonded NH₄+ ions. In Russian hydrothermal synthetic emeralds the strong infrared spectral features related to molecular water are generally similar to those of some natural emeralds but there are also broad bands due to Ni²⁺ and Cu²⁺ ions in the near-infrared regions which enable one to distinguish Russian hydrothermal synthetic from natural emeralds. Other bands unrelated to water in hydrothermal synthetic emeralds are also discussed.

A new definition of optic axis for gemmology

Richard H. Cartier, pp 228-234

Longstanding misuse of terminology in gemmological literature and course notes has allowed an inaccurate definition of optic axis to confuse understanding of this simple and fundamental concept. The problem is addressed with the proposal of a succinct new definition for optic axis. A review of anisotropic optics and discussion of the four kinds of optic axis explains the proposed definition accords with optics theory.