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(Follow this link for abstracts of past issues)
RARE GEM MINERAL DEPOSITS
FROM BRAZIL
PART 2: LAZULITE AND SCORZALITE
Dr Mario Luiz de Sá C. Chaves1, Dr Joachim Karfunkell1,
Dr Adolf Heinrich Horn2,
Dr. Donald B. Hoover3
1 Department of Geology and Museum of Natural History,
Geosciences Institute of the Federal University of Minas Gerais, Belo
Horizonte, Brazil.
2 Department of Geology, Geosciences Institute of the Federal
University of Minas Gerais,
Belo Horizonte, Brazil.
3 Hoover Associates, Springfield, USA.
ABSTRACT
The lazulite group minerals, lazulite and scorzalite,
constitute a complete solid solution series between the magnesium and iron
end members, much as with forsterite and fayalite in the olivine group. The
lazulite group minerals are valued as gemstones principally because of their
fine blue colour; but their relatively low hardness, and rarity in gem
quality make them of interest primarily to the rare gem collector. Brazil is
a major source of gem lazulite, but little is available in the literature on
sources in the country. This paper identifies the deposits and regions in
Brazil where lazulite group minerals occur, discusses their genesis, and
gives details of their gemmological characteristics.
LIMITATIONS OF THE PAVILION DEPTH RAPID SIGHT ESTIMATES METHOD
Garry Holloway
Precious Metals, Melbourne
ABSTRACT
This paper will show that the commonly used
method of pavilion depth estimation results in pavilion depths being over
estimated when table sizes are larger, and underestimated when crown angles
and girdles are steeper and thicker respectively.
RELATIONSHIP BETWEEN THE GROOVE DENSITY OF THE
GRATING STRUCTURE AND THE STRENGTH OF IRIDESCENCE
IN MOLLUSC SHELLS
Y. Liu1 K.N. Hurwit2
, Dr L. Tian3
1 Liu Research Laboratories, South EI Monte, California,
USA
2 Gemological Institute of America, Carlsbad, California, USA
3 Shandong Analysis and Test Center, Jinan, Shandong, China
ABSTRACT
The iridescence of mollusc shells and pearls is
caused by diffraction. A previous study showed that the strength of
iridescence colour of the shell of the pearl-producing mollusc Pinctada
margaritifera, and their groove density, are qualitatively related (Liu et
al. 1999). The groove density of a shell of the pearl-producing mollusc
Pinctada maxima is determined and a relationship between its groove density
and the strength of iridescence of its shells is investigated. The groove
density of the P. maxima shell is in between the groove densities of the
outer and inner surface of the shell of P. margaritifera. The strength of
the iridescence of this shell is also in between that of the iridescences of
its outer and inner surface. This study concludes that the strength of the
iridescence of the shell is again directly related to the groove density of
the reflection diffraction grating structure and the optical quality of that
grating structure. The iridescence of the shells is not related to the
species of mollusc.
AN UNUSUAL CRYSTAL FROM THE CENTRAL QUEENSLAND SAPPHIRE FIELDS
Roy Beattie
Brisbane, Queensland
ABSTRACT
This paper describes the features of a rare
bicoloured ruby-sapphire pyramidal crystal from the Subera mine, east of
Rubyvale, central Queensland.
GEM QUALITY PETALITE FROM MYANMAR
(BURMA)
U Kyaw Khaing Win, Ted Themelis
Yangon,,Burma
ABSTRACT
This brief report describes the occurrence and
identifying gemmological properties of crystals of colourless petalite that
have recently been found in a pegmatite at Khet-chal, Molo District (Momeik,
Myanmar).
LARGE OPL
™ DIFFRACTION GRATING SPECTROSCOPE
G.A.A. Instrument Evaluation Committee
Report
T. Linton, A.Cumming, K.Hunter
ABSTRACT
This larger version of the smaller, well known
OPL™ diffraction grating spectroscope can assist users due to its
high resolution and the wide linear spectrum it produces. This instrument
has a fixed slit width and a fixed focus, as does the smaller version of
this spectroscope. The Committee will comment on problems associated with
spectroscopes with these fixed features. This new instrument is easy to use
and it does resolve finer spectral features than those visible with the
small OPI, diffraction grating spectroscope-and, indeed, many other
spectroscopes including some hand-held prism spectroscopes. This report does
contain some information that may assist some gemmologists who have problems
with visual acuity when they attempt to observe fine line spectra that
characterise some gemstones.
DIAMOND CUT GRADING BASED ON RED REFLECTION INSTRUMENTS
Trevor Linton1, Carry Holloway2
1 Linton Enterprises, Wellington Point, Queensland
2 Precious Metals, Canterbury, Victoria
ABSTRACT
This paper compares four diamond cut grading
instruments and their methodologies. Each instrument is based on a red
reflector surrounding a lens. The instruments, the Brandt loupe™, the
Shindler loupe™, the Fire Scope™ and the IdealScope™
are compared by analysing four diamonds of differing proportions. As new
information revealed by recent computer-based diamond cut studies has a
bearing on the performance of these instruments, this is also discussed.
