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Sillimanite Gemstone Information

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About Sillimanite - History and Introduction

Sillimanite is an extremely rare and lesser-known collector's gem. Sillimanite is composed of aluminum silicate, which is a polymorph; sillimanite, kyanite and andalusite all share the same chemical composition (Al2SiO5), but each exhibit differing unique crystal structures. Sillimanite forms as orthorhombic, hexagonal crystals, whereas kyanite and andalusite crystals are both formed as triclinic crystals. Sillimanite is the rarest of these three closely related materials. Kyanite is known for its varying hardness and it also forms at much higher temperatures and pressure than sillimanite. Like andalusite, sillimanite may also exhibit pleochroism, but not nearly as pronounced as it is seen in andalusite.

Sillimanite is also the only alumino-silicate that is known to occur with cat's eye chatoyancy. Cat's eye chatoyancy is an extremely rare optical phenomenon known to occur in only a handful of gem-quality materials. It is a remarkable trait that is highly valued and sought-after by both gem collectors and jewelry designers. In cat's eye gems, the light reflects in such a way that when a gem is properly oriented and viewed under strong lighting, a light reflection reminiscent of the slit eye of cat appears to float or glide across the surface of the stone. The cat's eye reflection is owed to fibrous needle-like inclusions that are perfectly aligned in parallel patterns. Fibrous cat's eye sillimanite can exhibit obvious twisting of fibrous bunches, often without the need for magnification. This fibrous variety of sillimanite may be marketed as 'fibrolite', though it is not a trade name that is commonly used except by collectors.

Owing to its remarkable resiliency and resistance to high heat and chemical corrosion, sillimanite is an ideal material for both the gem and jewelry trade, as well as various industrial uses. For many years, sillimanite was a material frequently used for the making of tools by Native Americans. Since sillimanite is quite hard and durable, it is an excellent material to use for shaping other materials; and because of its ability to retain its strength even at very high temperatures, sillimanite is an ideal material for alumina refractories, though other, less expensive materials such as dumortierite and mullite (porcelainite) are generally favored for mass commercial production because good quality sillimanite comes at a much higher price. In addition to sillimanite's use for the production of alumina bricks, cement, ceramics and fine porcelain, sillimanite is also used for steel and iron smelting, as well as the manufacturing of industrial strength glass.

Faceted Sillimanite
Identifying Sillimanite Back to Top

Sillimanite occurs in colors very similar to a variety of other gemstone minerals. It is however, very unique with regard to its related polymorphs of andalusite and kyanite. Andalusite displays strong trichroism and sillimanite rarely occurs in blue like kyanite. Sillimanite's hardness is similar to that of quartz and garnet so it can hard to distinguish based on scratch testing alone. However, with the combined gemological properties of hardness, fracture, tenacity, density and refractive index, sillimanite can be easily identified using basic laboratory equipment. Cat's eye scapolite may be confused with cat's eye sillimanite, but sillimanite will usually exhibit a strong purple hue, whereas scapolite is typically more greenish to gray or brown in color. Compared to scapolite, sillimanite is significantly harder; scapolite has a score of only 5.5 to 6 on the Mohs scale. When compared to other cat's eye gems, sillimanite chatoyancy is quite pronounced, with good quality specimens exhibiting sharp and distinguished eyes, even when viewed under faint or indirect light sources.

Sillimanite Origin and Sources Back to Top

For many years, sillimanite was considered rare and was found in only a few regions of the world. The most important sources were known to come from Sri Lanka (Ceylon), Myanmar (Burma), Kenya and also the United States. With a recent find made in India, sillimanite has now become less of a rarity. Even so, it is still a collector's gem to this day and very few people who are not in the gem industry would recognize the gemstone by name. Modern day sources for sillimanite include European regions, as well as all of the previously mentioned sources, with most of the material on the market today coming from India. The US State of Delaware declared sillimanite as its official state mineral, even though it was actually discovered in the state of Connecticut. Other sillimanite sources in the United States include Wyoming and South Carolina. The Mogok region of Burma and the gem gravel sources of Sri Lanka were known to produce excellent quality material, including a rare light-blue and colorless variety of sillimanite. Transparent sillimanite is extremely rare, especially colorless sillimanite, since almost all material contains some impurities which cause the coloring of gemstone-quality deposits.

Buying Sillimanite and Determining Sillimanite Value Back to Top

Sillimanite Color

Sillimanite is almost always grayish to brown in non-gemstone deposits. Rarer colors, which are often used as gemstones include bright yellow, greenish-yellow, light-bluish, and violet to purplish-brown. Transparent sillimanite is usually greenish to yellow, and in rare cases, colorless and light-blue. Translucent to near-opaque gemstones are normally dark-grayish to greenish-brown. Some sillimanite will exhibit multiple colors depending on the angle from which it is viewed. Although its pleochroism is not as pronounced as in kyanite or andalusite, fine sillimanite will often flash yellow, green and blue from different angles. This can be seen more often in faceted gemstones than in cabochon sillimanite gems. Cat's eye sillimanite is almost always brownish in color, but high quality gemstones can exhibit a very strong violet hue.

Sillimanite Clarity and Luster

Gem-quality sillimanite is usually translucent and is very rarely found in transparent form. In thin slices, even sillimanite that appears opaque is actually translucent, allowing some light to pass through. Cat's eye sillimanite is usually translucent, but appears opaque due to the thick depth and orientation required to maximize its desirable cat's eye effect. In gemstones with fibrous formation, sillimanite may be bought and sold under the trade name of 'fibrolite', which appears as visibly twisted fibrous material. It is the fibrous form that is most often found with chatoyancy (the cat's eye effect). When cut and polished, sillimanite is known for its attractive luster which can range from vitreous (glass-like) to silky.

Sillimanite Cut and Shape

Sillimanite is a rarely faceted, since transparent material is extremely rare. Transparent sillimanite materials are normally yellow to green-yellow and are almost always faceted since the material is highly desired by collectors. In most cases, sillimanite is not cut with the intention to be used in jewelry; rather it is cut for showcase only, even though it is perfectly suitable for most jewelry designs since it has good durability, comparable to that of quartz. Translucent to near-opaque sillimanite is usually grayish to violet-brown in color, and those with fibrous parallel-aligned inclusions will be given the traditional cat's eye cut; en cabochon. Ovals are most common as they tend to preserve the most carat weight and with rare gemstone materials, it is an important aspect to consider before cutting. Experienced gem cutters will take extra care to ensure the cat's eye chatoyancy is centered and they will orient to maximize the chatoyancy so that it becomes visible under the faintest lighting. Almost all sillimanite cabochons will be cut with a high dome and the bottoms will normally be left unpolished or just slightly polished to allow for the bottoms to be glued to settings for rings and other jewelry designs. Calibrated gemstones are very rare, especially in matched pairs or sets, but standard-sized ovals can be found. Cabbed sillimanite gems will usually be a little uneven, which is considered normal unless the overall shape is beyond reasonable.

Sillimanite Treatment

Sillimanite is not known to be treated or enhanced in any way. However, sillimanite may be imitated by other, less valuable gemstones. Imitation gemstones may be dyed or heated to obtain the same silky luster and attractive colors of sillimanite, which includes both faceted gemstones and cat's eye cabochons. Sillimanite is not known to be treated or enhanced in any way. In rare cases, faceted sillimanite may be dyed to imitate other more precious gemstones, including sapphire, ruby or emerald.

Sillimanite Cat's Eye Gemological Properties: Back to Top
Chemical Formula: Al2[O/SiO4] - Aluminum silicate
Crystal Structure: Orthorhombic - hexagonal (usually fibrous)
Color: Light green, gray-white, yellow, brown, blue-green, blue, violet-brown (cat's eye). Rarely, light-pink and colorless
Hardness: 6.5 to 7.5 on the Mohs scale
Refractive Index: 1.655 to 1.684
Density: 3.23 to 3.27
Cleavage: Perfect
Transparency: Transparent to translucent - rarely opaque
Double Refraction or Birefringence: 0.014 to 0.021
Luster: Vitreous
Color of Streak White

Please refer to our Gemstone Glossary for details of gemology-related terms.

Sillimanite: Related or Similar Gemstones Back to Top
Andalusite Gemstone

As a polymorph of both kyanite and andalusite, sillimanite is closely associated with a variety of different gemstones. All three gems are members of the alumino-silicate series of minerals. Kyanite is a violet blue variety which forms with triclinic crystals. Both andalusite and sillimanite form with orthorhombic crystals but sillimanite is usually more fibrous and prismatic. Fibrolite is a trade name used for one fibrous variety of sillimanite which forms with fibrous inclusions. The different crystal structures for each of the different aluminum silicates is owed to different geological conditions during formation. Kyanite forms at a much lower temperatures and pressure than sillimanite, and sillimanite does not exhibit the strong pleochroism that both andalusite and kyanite possess. Scapolite is often mistaken for sillimanite as they have very similar properties, and they both are known to form with cat's eye chatoyancy. However, scapolite has a lower Mohs hardness level than sillimanite.

One variety of sillimanite is known to form with cat's eye chatoyancy, which is a rare optical phenomenon known to occur in only a handful of gem types. Tourmaline, chrysoberyl, sapphire (corundum), quartz and various varieties of garnet can often found in similar colors as sillimanite. Cat's eye quartz can often be mistaken for sillimanite, though sillimanite will usually exhibit more of an obvious violetish hue, while cat's eye quartz is more gray to brownish in color.

Most Popular Related Gemstones:

Kyanite, andalusite, tourmaline, chrysoberyl, chrysoberyl cat's eye and cat's eye quartz are the best-known associated gemstones. Kyanite and andalusite share the exact same chemical composition, but each possess uniquely different crystal structures. Other cat's eye gemstones include blue beryl (aquamarine), scapolite and apatite (as well as chrysoberyl and quartz).

Lesser Known Related Gemstones:

Biotite, feldspar (plagioclase, oligoclase, orthoclase, potassium feldspar etc.), tremolite, chrysotile, dumortierite and mullite are all also associated with sillimanite, but are not as well-known by the general public.

Sillimanite Mythology, Metaphysical and Crystal Healing Properties Back to Top

Sillimanite is usually more popular with collectors rather than crystal healers and metaphysical gemstone advisors. It is not an official birthstone, nor does it have any astrological associations like many other popular gemstones on the market today. However, there are legends and beliefs surrounding sillimanite, but most are associated with the cat's eye or fibrous variety of sillimanite. Sillimanite is believed to stimulate the production of natural endorphins responsible for pleasure, so it is thought of as a "feel good" crystal. It is even thought to help those who are stuck in their lives with daily tasks and routines, by enhancing the energy vibrations that help you focus on those things that are most important in one's life. For chakra energies, it is said to be powerful in supporting the heart and throat chakras and is said to enhance the intellectual spiritual body.

In general, cat's eye gems can be used as stones for looking into one's past, as well as for insight into the future. For those who wear cat's eye gems, it is believed that the eye is protective and acts as a symbol of the past, present and future. Cat's eyes are also believed to be beneficial for those with any eye-related health problems. If used and worn correctly, it is said to help alleviate problems with glaucoma, cataracts and vision problems such as near or far-sightedness.

In antiquity, as well as in the Middle Ages, people believed that the cosmos is reflected in gemstones. The esoteric movement revived the ancient belief and the gem industry made it another marketing tool to promote certain gems. Although the healing powers of gems remain a controversial issue, it has been mentioned for centuries by healers, shamans and medicine men of various cultures. Whether it's fact or a placebo effect, it truly doesn't matter as long as it helps those who need and believe in it. The best practice for those who use gemstones for crystal and metaphysical healing is to wear the gemstone or gemstone jewelry so that it is in direct contact with the skin, especially close to the troubled or wounded parts of the body.

Disclaimer: Metaphysical and Alternative Crystal Healing Powers and Properties are not to be taken as confirmed advice. Traditional, Ceremonial and Mythological Gemstone Lore is collected from various resources and does not represent the sole opinion of SETT Co., Ltd. This information is not to replace the advice of your doctor. Should you have any medical conditions, please see a licensed medical practitioner. GemSelect does not guarantee any claims or statements of healing or astrological birthstone powers and cannot be held liable under any circumstances.
Sillimanite Gemstone and Jewelry Design Ideas Back to Top

Sillimanite is a rather rare gemstone, and even rarer when it comes to jewelry. Faceted, transparent sillimanite is especially rare but with its decent hardness, it is perfectly suitable for most jewelry designs, whether it is an everyday-wear ring, or as an occasionally worn fashion jewelry design. Due to its rarity, you will not likely find sillimanite gemstones or sillimanite jewelry in your local retail jewelry store. Buyers seeking quality sillimanite will need to look for sillimanite via online dealers that specialize in colored stones. Sillimanite gemstones can make exquisite jewelry, particularly the cat's eye variety of sillimanite, which is ideal for rings and other designs which allow light to strike the gem directly. Rings will show off its strong chatoyancy better than earrings, as these designs tend to hit the light more, whereas necklace pendants may be hidden by hair or clothing when worn. However, sillimanite exhibits a rather strong level of chatoyancy, similar to chrysoberyl, so even when exposed to indirect lighting, fine cat's eye sillimanite can reflect a cat's eye. Faceted sillimanite looks exquisite in any settings, and when cut properly, sillimanite can exhibit excellent reflections; bright, vivid and attractive sparkle that challenge even the finest sapphire and golden chrysoberyl.

Note: Always buy colored gemstones by millimeter size and not by carat weight. Colored stones can vary tremendously in regard to size-to-weight ratio. Some stones are often much larger, while other types of stones may be much smaller in size when compared to a diamond of the same carat weight.

Sillimanite Gemstone and Jewelry Cleaning and Care Back to Top

How to Clean your GemstonesSillimanite is rather durable with a hardness similar to that of quartz. However, with its perfect cleavage, uneven fracture, and brittle tenacity, it can be somewhat prone to chipping if not cared for when worn as jewelry. Since sillimanite is usually cut en cabochon, this normally is not too much of a concern, but when faceted, sillimanite can wear down along the edges. To clean your sillimanite gems and jewelry, it is recommended that only plain soapy water and a soft cloth are used. Be sure to rinse gemstones well to remove soapy residue, and while cleaning, it is best to avoid the use of any harsh chemicals or cleaners, such as bleach or ammonia. As with all colored stones, it is best to avoid ultrasonic cleaners and steamers as they can damage the color, or even cause the stone to fracture.

Always take off jewelry before exercising, playing sports or engaging in chores, such as gardening or washing dishes. Store sillimanite gems away from other gems and jewelry to avoid scratches and fractures. It is best to always wrap your gemstones in soft cloth or place them inside a fabric-lined jewelry box for extra protection.

  • First Published: July-29-2015
  • Last Updated: November-08-2018
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