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
SPECIE
Sillimanite
Composition:
Al2SiO5
Varieties:
transparent by color, translucent to opaque, and chatoyant
Colors:
green, yellow, brown, black, grey, blue, white, colorless
Phenomena:
chatoyancy
Streak:
white
Crystal System
orthorhombic
Habit:
usually fibrous massive crystals; long prismatic crystals are rare
Cleavage:
perfect in one direction
Fracture:
uneven and brittle
Fracture Lustre:
vitreous pearly
Lustre:
silky vitreous 
Specific Gravity
varies 3.23-3.2; compact mineral 3.14-3.18
Hardness
6.5-7.5
Toughness:
poor
Refractive Index
alpha 1.654-1.661; beta 1.658-1.662; gamma 1.673-1.683
Birefringence:
0.020
Optic Character
biaxial positive
Dispersion:
0.015
Pleochroism
pale brown, yellow green-brown, green-brown, blue
Ultraviolet 
Flourescence
Sri Lanka: inert; Burmese: blue-weak reddish
Spectra
Sri Lanka: weak line 4100; sharp lines 4410 & 4620
Color Filter
no information
Solubility
no reaction to acids
Thermal Traits
infusible; avoid thermal shock
Treatments
none
Inclusions
fibers