Lets Talk Gemstones
By Edna B. Anthony, Gemologist
Please contact the author for permission
to reprint this article.
Dioptase: A Cyclosilicate
Dioptase incorporates the
native metal, copper in the Si6O18 ring structure with water to form the
only gem material that so closely approaches the finest color of emerald.
The small and well-defined deep green rhombohedral crystals develop in
oxidation zones of weathering copper ore deposits. Its growth in voids
of dolomite and limestone formations and irregular druse cavities is often
in association with malachite and smithsonite, but admixtures and inclusions
seldom occur.
Alluvial finds in nodular
form attest to its chemically stable nature and its resistance to corrosion.
Crystals exhibit characteristic dense striations of the rhombohedral faces.
Numerous readily visible internal cleavage fractures frequently impart
a pearly lustre and cause unusual reflections from within the transparent
stones. This dictates the use of only the purely transparent ends of larger
crystals for faceted gemstones and also presents difficulties in making
table and step cuts. Thus, gems of more than a carat are extremely rare.
Cabochons are sometimes fashioned from compact fine-grained translucent
nodules.
According to Jaroslav Bauer
and Vladimir Bouska in A Guide In Color To Precious And Semi-Precious Stones,
the first known crystals of dioptase were brought to Semipalatinsk in Kazakhstan
from the Kirgiz Steppes in Siberia in 1870 by the Bukharan merchant Akhir
Mahmed. They were found on the western slopes of the Altai Mountains on
calcite deposited in the numerous crevices of the limestone hill of Altyn-Tyube.
Specimens sent to the Academy in St. Petersburg were identified by member
German mineralogist R. Ferber as an emerald variety and named "achrite".
Its low hardness made it less valuable than emerald, and it became known
as "emerald for the poor". When it was determined in 1801 to be a new mineral,
the French mineralogist Rene Just Hauy used the Greek words dia (through)
and optasis (vision) to create the name dioptase. This describes the visible
reflections on the internal cleavage cracks. "Copper emerald" is a name
also used by some collectors of mineral specimens, and this term appears
in almost all collections.
The discovery of some fairly
large worn crystals extracted from the gold-washings in the Yeniseisk area
and the failure to discover other deposits led to the assumption for many
years that Siberia was the only source of the mineral dioptase. Fine cuttable
dioptase material now comes from Reneville in Zaire's Congo. The Otavi
Range at Guchabo in Namibia produces small amounts of dioptase. Pinal County,
Arizona in the USA, Copiapo in Chile, Baita in Romania, and Peru are other
world sources for dioptase. Despite its beauty, fragile dioptase is not
suitable for use in jewelry.
The Mineral Museum at the New
Mexico Institute of Mines and Technology in Socorro showcases a magnificent
dioptase specimen that is worth the drive to Socorro to view it. Many other
mineral museums have wanted this particular dioptase specimen for their
collections, but, hopefully, it will remain in Socorro.
More on Cordierite
In regards to my cordierite
article in the January/February 1998 issue of the New Mexico Facetor, I
wish to clarify the spelling on both "praseolite" and "prasiolite". According
to An Illustrated Dictionary of Jewelry by Harold Newman, "praseolite"
is a variety of iolite that is leek-green. From the same source, "prasiolite"
is a variety of amethyst from Montezuma, Brazil that has been changed by
heat-treatment to a leek-green color. Newman says that the name is deprecated,
owing to confusion with "praseolite", which was earlier so named. A variety
of amethyst discovered in Arizona has been similarly changed by heat. The
"e" in "praseolite" denotes the association with iolite, while the "i"
in "prasiolite" applies to quartz. To add even more confusion to this topic,
we also have "prase", the cryptocrystalline translucent leek-green variety
of quartz.
It is interesting to note
that the prefix "pras" originates from the Greek word "prason", which means
leek. This term has been used in conjunction with several varieties of
gemstones, including prasopal.
Gemstone Properties
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Cu6(Si6O18)6H2o (a
hydrous copper silicate)
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intense green and blue-green |
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transparent to translucent |
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stubby six-sided columns
and compact fine-grained nodules |
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one perfect (Joel Arem);
three perfect (Richard T. Liddicoat) |
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conchoidal, uneven, and
brittle |
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very poor; brittle with
cleavage |
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0.022 (Walter Schumann),
0.036 (Joel Arem); 0.028 (Jaroslav Bauer and Vladimir Bouska) |
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weak emerald green-blue
green |
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strong absorption of violet
and blue; band at 5500 |
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rapid etching/dissolution
in HCL, HNO3, and ammonia |
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rare; darker color caused
by traces of iron |