Let's Talk Gemstones:
By Edna B Anthony, Gemologist
Please contact the author before reprinting this
article.
Sillimanite
This article marks the third and last in my series on
the polymorphic aluminum silicate group. This group is comprised of andalusite,
kyanite, and sillimanite. Sillimanite was named in honor of the American
mineralogist Benjamin Silliman (1779-1864), who was a chemistry professor
at Yale.
Sillimanite is seldom transparent. However, a deposit
in Mogok, Myanmar (formerly Burma) yielded lovely and rare violet-blue
facet-grade crystals. The pale blue to colorless material from Kenya matches
the Burmese material in quality, but the crystals from Burma tend to be
smaller in size. The gem gravels of Sri lanka yield rare greyish-green,
transparent, and fibrous chatoyant stones. Collectors search for these
transparent varieties of sillimanite because the material is so rare.
Sillimanite occurs in deposits found world-wide. The
many sources for sillimanite include Tanzania, South Africa, Korea, India,
Madagascar, Brazil, France, Germany, Italy, Ireland, Scotland, Canada,
and the United States. The producers of sillimanite in the eastern United
States include North Carolina, South Carolina, Delaware, Pennsylvania,
Connecticut, and New York. Sillimanite is also found in Oklahoma and South
Dakota. The Clearwater River Valley in Idaho has sillimanite cobbles that
are carved into figurines and sold as souvenirs of Idaho.
Text written by Dr. J. Kourimsky, found in the Illustrated
Encyclopedia of Minerals and Rocks, states that sillimanite is "commonly
finely fibrous to acicular and is colored white; when mixed with quartz,
it is called fibrolite."
One of our prized reference books that Tony and I acquired
for my gem and mineral library is an old textbook entitled "Manual of Mineralogy",
19th edition, by Dr. Cornelius S. Hurlbut, Jr. and Dr. Cornelis Klein,
after J.D. Dana. I found the information and explanations from this book
to be very explicit, comprehensible, and thorough. The following sentences
quoted from this book will explain why research can be such fun for me.
"Sillimanite occurs as a constituent of high-temperature metamorphosed
argillaceous rocks. In contact-metamorphosed rocks, it may occur in sillimanite-cordierite
gneisses or quartz-muscovite-biotite hornfels. In regionally metamorphosed
rocks, it is found, for example, in quartz-muscovite-biotite-oligoclase-almandite-sillimanite
schists. In silica-poor rocks, it may be associated with corundum." What
a thoroughly descriptive mouthful that was!
Sillimanite, like kyanite, presents problems when rendering
faceted representations. The brittleness and directional cleavage of sillimanite
present challenges for faceters. The scarcity of facetable sillimanite
and its difficulty in cutting add value to a faceted sillimanite. The sillimanite
catseye cut en cabochon make unusual and very attractive stones for rings.
Gemstone Properties
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transparent by color,
translucent to opaque, and chatoyant |
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green, yellow, brown,
black, grey, blue, white, colorless |
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usually fibrous massive
crystals; long prismatic crystals are rare |
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varies 3.23-3.2; compact
mineral 3.14-3.18 |
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alpha 1.654-1.661; beta
1.658-1.662; gamma 1.673-1.683 |
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pale brown, yellow green-brown,
green-brown, blue |
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Sri Lanka: inert; Burmese:
blue-weak reddish |
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Sri Lanka: weak line 4100;
sharp lines 4410 & 4620 |
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infusible; avoid thermal
shock |
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