What's in Our Current Issue

Volume 23, Number 10, April - June 2009
Grahame Brown Memorial Issue - Part Two

Grahame Brown Memorial Issue - Part Two - Apr/Jun 2009  
  • The radiopacity of some common gem materials
  • Large brazilian diamonds
  • Optical phenomena in gemstones
  • Quahog pearls - amongst natures rarest beauties
  • A study of zircon from Tanzania - Malaya zircon
  • Meteorites: origins, properties and gemmological significance
  • Letter to the editor:
    Update on Chinese freshwater cultured pearls and their Raman spectra

(Follow this link for abstracts of past issues)

WELCOME TO PART 2 OF THE GRAHAME BROWN MEMORIAL ISSUES

It is appropriate that the first article in this issue is a reprint of the first paper that Grahame, together with dental colleague Alex Moule, and well known gemmologist Bob O'Neil published in November 1975. Volume 12 Number 8 of The Australian Gemmologist.

THE RADIO OPACITY OF SOME COMMON GEM MATERIALS

G. Brown, A.J. Moule, and R.L. O'Neil

LARGE BRAZILIAN DIAMONDS

Dr. D.B. Hoover FGA, FGAA (Hon.)1  and Dr. J. Karfunkel DGemG2

1 Hoover Associates, Springfield, Missouri, USA

2 Federal University Minas Gerais, Belo Horizonte, Brazil

ABSTRACT
 Many gemmologists are unaware of the large number of very large diamonds that have come from a small region of Minas Gerais state, Brazil, called the Alto Paranaiba. We provide a brief history of diamond production from the region, and a listing of some 65 stones over 50 carats from the region. Undoubtedly many more large diamonds have come from the region, but most production is spirited out of the country leaving little evidence that such diamonds ever existed. The region is still noted for production of fine pink and red stones.

Key-words: Diamonds, Brazil, Alto Paranaiba, Coromandel, Vargem. Kimberlites, garimpeiros.

OPTICAL PHENOMENA IN GEMSTONES

Richard Cartier

ABSTRACT
 Around twenty-five years ago I encountered a comment by Grahame Brown about an error in gemmological literature concerning the cause of the blue colour in the sheen of fine moonstone. Even though he did not have an alternative explanation to offer, he had the confidence to trust his own understanding, I and to point out what he saw as an
inconsistency. His emphasis on understanding encouraged me to critically consider published explanations of this and other optical phenomena, and was instrumental in motivating me to write my
book on optics.

Although "optical phenomena" covers a host of ideas including selective absorption and luminescence, here I shall discuss phenomena that may he described as a type of sheen. Much of the literature offers differing and sometimes conflicting uses of terms, and the following definitions and explanations are this author's attempt to rationalize them.

Key-words: phenomena, sheen, chatoyancy, asterism, iridescence, orient, adventurescence, adularescence, diffraction

 

QUAHOG PEARLS  - AMONG NATURES RAREST BEAUTIES

By Antoinette Matlins, PG
author of The Pearl Book: The Definitive Buying Guide

ABSTRACT
 Quahog pearls (pronounced koh' hog) are among the rarest and the most beautiful of natural pearls. While the finest seem to been have found off the North Atlantic coast of the United States, in the waters off Massachusetts and Rhode Island, they have been found as far south as Florida. Valued for centuries by Native Americans who lived along the New England coast, quahog pearls were used for self adornment and wampum, an early form of money. The name itself, quahog, is derived from the Algonquin Indian name for this particular clam, "Poquauhock", a common hard shell variety, and this accounts for why the word is seen spelled in several ways, "quahaug," "cohog," and "quohog."

Key-words: Pearl, quahog, clam, Venus mercenaria, wampum, Algonquin, non-nacreous..		  


Quahog Pearl

A STUDY OF ZIRCON FROM TANZANIA - MALAYA ZIRCON

Dr Ulrich Henn

German Gemmological Association
Idar-Oberstein, Germany

ABSTRACT
 The so-called Malaya zircons originate from the Tanga Province of Northern Tanzania. The stones show an attractive range of bright colours from yellow to brown and red. The Malaya zircons represent typical "high crystalline" zircons with high values of density and R.I. and a typical absorption spectrum with narrow bands and fine lines of tetravalent uranium.

Key Words: Zircon, Tanzania, Malaya, uranium, tetravalent.

A STUDY OF ZIRCON FROM TANZANIA - MALAYA ZIRCON

Dr Ulrich Henn

German Gemmological Association
Idar-Oberstein, Germany

ABSTRACT
 The so-called Malaya zircons originate from the Tanga Province of Northern Tanzania. The stones show an attractive range of bright colours from yellow to brown and red. The Malaya zircons represent typical "high crystalline" zircons with high values of density and R.I. and a typical absorption spectrum with narrow bands and fine lines of tetravalent uranium.

Key Words: Zircon, Tanzania, Malaya, uranium, tetravalent.

METEORITES: ORIGINS, PROPERTIES AND GEMMOLOGICAL SIGNIFICANCE

By Grant Pearson

ABSTRACT
 Meteorite materials have been of interest since antiquity for ornamentation or amulets, or as objects of fear, curiosity, worship or veneration. For example, the lack stone of Kaaba in Mecca to which pilgrims have long paid homage is thought to be meteoritic with its ancient mystical allusions probably arising from its fallen from the heavens. The stony grey meteorite weighing 127 kilograms that fell in Ensisheim in Alsace (1492) is revered as a local treasure and token of good fortune, and a meteoritic mass that was worshipped in Ephesus as an Earth Goddess is described in the biblical Acts Of The Apostles. The word "meteorite" is considered to come from the Greek "met-ora" or "things in the air" and something of the order of 500 or so significant meteorites are thought to strike the planets surface each year, although only a very few of these are recovered.

Key Words: meteorite, widmannstatten, octahedrite, pallasite, olivine, tektite, moldavite, chondrite, kamacite, taenite.

 		  Wolf Creek crater, Tanami, Western Australia.
Wolf Creek crater, Tanami, Western Australia.

LETTER TO THE EDITOR:
UPDATE ON CHINESE FRESHWATER CUTURED PEARLS AND THEIR RAMAN SPECTRA

Dr. Stefanos Karampelas
Gübelin Gemmological Laboratory
Maihofstrasse 102, CH-6006, Lucerne 9, Switzerland