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History & Introduction


Sillimanite is an aluminum silicate that relates to both kyanite & andalusite. In fact these three minerals are polymorphs -- they share the same chemical composition but different crystal structures. The formation of Sillimanite is under higher temperatures & immense pressures.

Sillimanite is named after the American chemist Benjamin Silliman (1779-1864). Silliman is actually the important person the chemist and geologist who discovered the constituent elements of many minerals. Geologically, sillimanite has a hardness of 6.5 to 7.5 with a vitreous pearly luster. Sillimanite is available in many colors, ranging from colorless to gray-white, bluish yellowish and brownish. Sillimanite even found in fibrous form sometimes known as fibrolite, it is named as such due to the mineral appears like a bunch of fibres twisted together.

Historically, sillimanite is a rare gemstone, known mainly to collectors. Compact masses of Sillimanite were once used by Native Americans of the American Southwest as fashion tools. Sillimanite is considered to stimulate the production of endorphins in the human body. It is even believed increasing energy and a general feeling of well-being. It supports in self-confidence and self-belief and self-love. The stone acts as healing powers for the areas of the throat and the heart. It is also the astrological stone for the sign of Aries.

Recognize - Aluminum and silicate together form sillimanite. It can run from transparent to translucent and features a silk-like, glassy texture. Gem quality sillimanite crystals are rare and is seen seldom at retail. For its fibrous inclusions, it is sometimes also known as "fibrolite" in the jewelry world. The hardness of the stone is 6 - 7 on Mohs scale with 3.23 as specific gravity.

Availability - A third variety, usually used in the creation of artifacts, is originated in the Clearwater River Valley in Idaho. The rare transparent Sillimanite crystals from Sri Lanka, India and Burma are termed to be important rare collector stones, and cut rarely into beautiful gemstones for collectors. Sillimanite is currently found in Burma and Sri Lanka. It is even mined here in the United States in the states of Delaware, Idaho and Massachusetts.

Unusual prismatic and waterworn transparent crystals of Sillimanite come from Mogok, Burma (Myanmar); even from the Ratnapura District situated in Sri Lanka. Orissa, India, is another uncommon source for transparent crystallized Sillimanite.

In the U.S., brownish fibrous crystal groupings of Sillimanite come from Chester, Middlesex Co., Connecticut, Willimantic, Windham Co. and Connecticut, New London Co.; where small crystals in matrix are available. Sillimanite in Fibrous masses are available in Brandywine Springs, and dark brown crystals crystal sprays from South Carolina. Gray Sillimanite masses are available in Wyoming.

Usages - Sillimanite is mainly used along with the formation of mullite refractory ceramics, high temperature crucibles and spark plugs. For jewelry purposes the chatoyant green and blue varieties are used. It is also used as an ornamental stone.

Sillimanite is sometimes used to imitate other precious gems like rubies, emeralds, sapphires and that are similarly fibrous in nature. Caution should always be used in purchasing gemstones to make certain that you know exactly what you are purchasing.

Buying Guide

Color - Sillimanite is formed from aluminum silicate and is a transparent to translucent mineral ranging from brown, white to gray and green. Blue variety of sillimanite is the most sought.

Clarity and Lustre - If it is not fibrous, excellent field indicators are the crystal habit, color, brittleness and hardness. It is mostly found as silky and fibrous masses with glassy luster.

Cut and Shape - The stone is relatively hard, measuring in at between and seven and eight on the Mohs scale of hardness. It facets beautifully but can also be cut into cabochons or made into beads.

Gemological Characteristics:

Chemical Formula Al2SiO5, Aluminum silicate
Color Brown, yellowish-gray, gray, white, light pink, or light purple
Streak Colorless
Hardness 6.5 - 7.5
Crystal System Orthorhombic
Crystal Forms and Aggregates Frequently fibrous, sometimes with radiating crystal sprays embedded in matrix. Even columnar, massive, compact, and as rounded water worn crystals and crystal masses. Prismatic crystals are found occasionally.
Transparency Transparent to translucent
Specific Gravity 3.2 - 3.3
Luster Vitreous, silky
Cleavage 1,1
Tenacity Brittle
Rock Type Metamorphic
Popularity (1-4) 3
Prevalence (1-3) 3
Demand (1-3) 2

Classification:

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