• Dispersion Measurement with the Gemmologist's Refractometer - Pt 2

• Brandt Proportion Loupe

• Rare Australian Ornamental Materials - Zebra Rock

• Staurolite from the Lai Hka and Mong Keng Areas, Myanmar

• Identification of GE POLÔ Diamonds

(Follow this link for abstracts of past issues)

DISPERSION MEASUREMENT WITH THE 
GEMMOLOGIST’S REFRACTOMETER - Part 2

D.B. Hoover
Springfield, Missouri, USA
Trevor Linton
Wellington Point, Queensland, Australia

ABSTRACT
Part 2 of this paper gives details of the procedures the authors used to obtain an empirical calibration of two commercial critical angle refractometers. This involves the selection of light sources, linearization of dispersion curves by the Sellmeier equation, and details of the measurement process.

  In the course of their research, the authors recognized problems with literature values of the dispersions for many gem materials. These problems are reviewed, and comparisons made with the various literature sources of dispersion measurements and compilations. The review suggests that a thorough re-examination of gemstone dispersion data is needed.

BRANDT PROPORTION LOUPE

T. Linton, A. Cumming (Queensland)
N.Masson (Victoria)
B. Sweeney (Tasmania)

ABSTRACT
Rapid sight estimation methods for assessing cut proportions at 1-x magnification were first described by Richard Liddicoat in 1962. The rapid sight method for estimating a round brilliant diamond’s table size, pavilion depth, and total depth as a percentage of the table diameter is based on the particular cut’s capacity to provide a symmetrical pattern of reflections from the table and crown facets in the pavilion facets. The visibility of this pattern of light and dark reflections can be enhanced by viewing these reflections as red-enhanced images.

Selwyn Brandt, of The House of jewellery, markets a Triplet Proportion Loupe that produces red-enhanced images of these reflections. This is achieved by a modified 10x hand lens that creates red reflections of the table and crown facets in the pavilion facets of diamond and so makes the interpretation of Liddicoat’s rapid sight methods for assessing diamond cutting quality that much easier to achieve.

Rare Australian ornamental materials - ZEBRA ROCK
AN ORNAMENTAL STONE FROM THE 
EAST KIMBERLEY, WESTERN AUSTRALIA

Dr A.W.R. Bevan
Department of Earth & Planetary Sciences, 
Western Australian museum of Natural Science, Perth

ABSTRACT
A distinctive reddish brown and white banded sedimentary rock from the east Kimberley of West Australia called zebra rock, otherwise known as zebra stone or sometimes ribbon stone, has found many ornamental uses since its discovery in 1924. Composed essentially of small particles of quartz and sericite’ (fine-grained white mica), zebra rock also contains the clay mineral kaolinite and its polymorph, dickite, as well as alunite. Recent work has shown that the colour banding of zebra rock probably formed by the rhythmic precipitation of iron oxide (hematite) rich bands during the alteration of the rock by percolating fluids.

  U Tin Hlaing
Department of Geology, Taunggyi University, Myanmar

ABSTRACT
Crystals of cabochon grade staurolite, from the ~1,900 km² belt of schists, located to the west of the townships of Lai Hka and Mong Keng in Myanmar’s Shan State, are described and illustrated. 

  E. Fritsch
University of Nantes, France
J.-P. Chalain, H. Hänni
SSEF – Swiss Gemmological Institute, Switzerland

ABSTRACT
Ge POLÔ damonds are bownish type 11a diamonds that have been converted into colourless to near-colourless diamonds by a secret high pressure-high temperature treatment. Identifying features of these colour enhanced diamonds include laser-inscribed girdles and transparency to short-wave untraviolet wavelengths. Indicators of HPHT treatment include cross-hatched tatami graining of weak to moderate intensity, generalised haziness, etched surface-reaching cleavages, and rare mineral inclusions surrounded by discoidal stress fractures. Confirmation of identity is possible with the us eof 514 nm laser-induced photoluminescence – as measured on a Raman spectrometer – to examine the fluorescence of this treated diamond’s N-V centre at low temperature. In HPHT treated GE POLÔ diamonds the 637:575 nm ratio is above 2.5 and lower than 1.6 in untreated diamonds. Also, the Full Width at Half Maximum (FWHM)  of the 637 nm band is higher in GE POLÔ diamond than in natural colour type 11a diamonds.