Sillimanite Gemstone

Sillimanite Gemstone
[For Information and Educational purposes only]
by Alice Koh

Sillimanite (also known as "fibrolite") is named after Benjamin Silliman (1779-1864). Silliman was the founder of the American Journal of Science (Silliman's Journal). He was an important chemist and also the person to distill petroleum.

Sillimanite is a polymorhph variety of aluminum silicate minerals that is related to both andalusite and kyanite. In fact, these three minerals share the same chemical composition (Al2SiO5) but have different crystal structures. Sillimanite forms as orthorhombic, hexagonal crystals, whereas kyanite and andalusite crystals are triclinic. Moreover, of the three alumino-silicate minerals, only sillimanite occurs with the cat's eye effect. Another difference is that sillimanite rarely occurs in blue like kyanite.

Sillimanite is named fibrolite because the mineral appears like a bunch of fibres twisted together. It is this fibrous form of sillimanite that is most often found with chatoyancy (the cat's eye effect). Chatoyancy or the cat's eye effect in a gemstone (especially when cut en cabochon) shows a band of bright reflected light caused by aligned inclusions in the stone. This bright reflected light is similar to the slit eye of a cat.

Sillimanite occurs in three coloured varieties: yellow, brown and blue. Bright yellow, greenish-yellow, light-bluish and violet to purplish-brown are rarer colours. Cat's eye sillimanite is almost always brownish and the high quality sillimanite gemstones will exhibit a very strong violet hue. The cat's effect in sillimanite is very sharp and distinguishable.

Sillimanite has a hardness of 6.5 to 7.5 on the Mohs scale thus making it suitable for most jewellery designs. When cut and polished, sillimanite is known for its attractive luster which can range from vitreous (glass-like) to silky. It has a refractive index of 1.655 to 1.684 and its specific gravity is 3.23 to 3.27. It has a perfect cleavage which means that care should be taken when setting and wearing sillimanite since a hard blow could cause it to split.

Fine sillimanite can exhibit a strong pleochroism of showing two or three colours when viewed from different angles. These fine specimens will often flash yellow, green and blue.

Sillimanite is hard and durable, ideal for jewellery. Native Americans used sillimanite for shaping other materials. Due to its remarkable toughness and resistance to high heat and chemical corrosion, sillimanite is used for the production of alumina bricks, cement, ceramics and fine porcelain, steel and iron smelting, as well as the manufacturing of industrial strength glass.

Ovals are the most common cut in sillimanite. Typically, sillimanite cabochons will be cut with a high dome with the bottom part left unpolished in order to be glued to jewellery settings. Sillimanite is not known to be treated or enhanced in any way.

Sillimanite imitations do exist and these have been dyed or heated in order to create the silky luster and similar colour appearance of sillimanite. Sillimanite imitations include faceted and cat's eye gemsones. Sometimes, faceted sillimanite is dyed to imitate precious gemstones such as sapphires, rubies and emeralds.

Cat's eye quartz can often be mistaken for sillimanite, though sillimanite will usually exhibit more of an obvious purplish hue, while cat's eye quartz is more gray to brownish in colour.

Sillimanite gems may appear similar to a variety of other stones, such as the colour change diaspore, scapolite, beryl, quartz and moonstone.

Cat's eye sillimanite is believed to stimulate the production of the happy chemical endorphins in the brain. In some circles, it is even called the "feel happy crystal."

According to crystal healing beliefs, sillimanite is a useful stone that can contribute to a positive mental attitude and give the wearer the energy to be productive.

Cat's eye gemstones are great for scrying, and to look back to our past and study it more profoundly, and or to have insights of the future. Cat's eye gemstones are powerful stones for protection.

Sillimanir is a collector's gemstone and is not known in general public. Thus it does not have any astrological associations nor assigned a birth month.

A rare light-blue and colourless variety of sillimanite is sourced from the Mogok region of Burma and Sri Lanka. The most important sources for sillimanite are Sri Lanka (Ceylon), Myanmar (Burma), Kenya, the United States and European regions. Recently sillimanite was found in India which hopefully soon it will become available for everyone. Sillimanite was discovered in the state of Connecticut, however, the US State of Delaware declared sillimanite as its official state mineral.

To clean your sillimanite gemstone and jewelry, it is recommended that only plain soapy water and a soft cloth be used.