SILLIMANITE (=FIBROLITE) Trimorphous with Kyanite, Andalusite.

Optics: a =1.654-1.661 β= 1.658-1.662; γ= 1.673-1.683.

Biaxial (+), 2V: 21—30°.

Occurrence: A mineral of metamorphic rocks, such as schists and gneiss; also granites.

Idaho; South Dakota; Oklahoma; Pennsylvania; New York; Connecticut; Delaware;  North Carolina; South Carolina.

Canada; Ireland; Scotland; France: Germany; Czech Republic; Slovakia; Brazil; India; Madagascar; North Korea; South Africa; Tanzania.

Sri Lanka and Burma: green, blue. violet—blue facetable material; also from Sri Lanka, grayish green, chatoyant fibrolite.

Kenya: facetable crystals, pale bluish color to colorless, S.G. 3.27.

Comments: The fibrolite from Burma and Sri Lanka is well known to gem collectors, and highly prized because of its great scarcity. Blue and greenish gems are lovely, although very difficult to cut. Chatoyant material sometimes yields catseye fibrolites, which are also very rare. The material from Kenya is just as attractive as Burmese fibrolite but seems to be somewhat smaller in size.

Name: After Benjamin Silliman, mineralogist, of Yale University. Fibrolite is in allusion to the fibrous nature of this variety.

Sillimanite Gem: Info on Properties, Meanings & Prices

Sillimanite is an aluminosilicate commonly sold in two forms: cat’s eye stones or faceted white, yellow, or green gemstones. Both forms come in various colors.  

While beautiful, sillimanite is also rare and difficult to cut, so it’s more well-known among collectors and jewelers. Along with the other minerals in the group (kyanite and andalusite), sillimanite is the rarest, making it a special piece in any display.

As of 1977, sillimanite serves as the official state mineral of Delaware, USA. Despite its rarity, sillimanite can be incredibly durable, making it great for everyday wear. 

Before you shop for sillimanite, check out this guide to learn all of sillimanite’s properties, value factors, prices, meanings, and more!

What Is A Sillimanite Gemstone?

Sillimanite is a glassy or silky semi-precious gemstone with a few historical names, including fibrolite, monrolite, bamlite, and bucholzite. Though its gemstone forms are gorgeous, 95 percent of sillimanite uses are industrial. 

What is sillimanite used for? The primary industrial use is for manufacturing refractory material like glass and ceramics. It’s also used for making metals and cement, along with smelting iron and steel.  

Throughout its history, sillimanite has also been used for creating tools from other materials. Some other products and uses for sillimanite include:

• Porcelain

• Industrial-strength glass

• Alumina bricks

Gemstone-wise, sillimanite serves as an alternative May birthstone. Cat’s eye sillimanite is also a planetary star stone for the moon. 

Sillimanite Specifications & Characteristics

Sillimanite’s mineral group also includes kyanite and andalusite. They’re all polymorphs (same composition, different crystal structure) with each other, though their appearances and formation conditions are distinct.   

This mineral is an aluminosilicate. Depending on the source, the chemical formula for sillimanite’s composition may be written Al2SiO5, Al2SiO3, or Al2(SiO4)O. Iron and titanium are common impurities.

The two general forms for sillimanite, raw, are fibrous or crystalline. Sillimanite can form compact or fibrous masses. Its crystals are prismatic and slender, but rarer than fibrous material. 

Here are the remaining sillimanite properties:

• Mohs hardness: 6.5-7.5

• Color: Colorless, white, gray, yellow, brown, green, blue, violet-blue

• Crystal structure: Orthorhombic

• Luster: Vitreous (glassy), sub-vitreous, greasy, or silky

• Transparency: Transparent to opaque

• Refractive index: 1.65-1.68

• Density: 3.23-3.27 - crystals; 3.14-3.18 - compact

• Cleavage: Perfect, 1-direction on [010]

• Fracture: Irregular/Uneven

• Streak: White

• Luminescence: Fluorescence in blue Myanmar stones - red in SW-UV & LW-UV

• Pleochroism: Sometimes present & strong in chatoyant material and blue or green stones; Chatoyant stones - light yellow, gray-brown, clove-brown; Blue stones - colorless, blue, and light yellow; Green stones - Dark green, yellow-green, blue 

• Optical effects: Rarely chatoyancy

Types of Sillimanite

The only variety of sillimanite is fibrolite, or sillimanite cat’s eye. But what is a sillimanite cat’s eye? 

Fibrolite (a.k.a. cat’s eye sillimanite or chatoyant sillimanite) is a fibrous sillimanite variety, hence the name. It displays chatoyancy (the “cat’s eye” effect), meaning parallel bunches of fibers align to reflect a ray of light down the stone’s center, resembling a cat’s eye. These stones can be transparent to opaque. 

A sillimanite cat’s eye gemstone almost always has a silver reflected light, but the body color differs. Reddish-black, dark brown, or grayish-green are most common, but it can also be:

• Red

• Yellow

• Brownish-orange

• Tan

• Black

• Blue (deep, light, or grayish)

Recently, rare violet-blue specimens from Sri Lanka and Myanmar have appeared on the market.

Sillimanite Gemstone Meaning & History

Sillimanite symbolizes happiness, success, and inner peace. It’s also believed to represent willpower and wisdom, especially for encouraging strong leadership traits.

Cat’s eye sillimanite’s meaning is associated with past-life exploration and foresight. These gems represent the journey of past, present, and future. They were historically believed to protect the wearer. 

History

The earliest evidence of sillimanite dates back to prehistoric times. Neolithic compact sillimanite fashioned into polished blades, similar to those made from nephrite jade, were uncovered underwater near France in 2010. Additionally, Native Americans have used sillimanite to make tools for a long time.

Sillimanite’s first name was Faserkeisel (German for “Fiber silica”), given by Austrian chemist Joseph Lindacker in 1792 for specimens found in Bohemia (present-day Czechia). 

In 1802, French mineralogist Jacques-Louis, Comte de Bournon named fibrous sillimanite material from southern India (the Carnatic region) fibrolite. 

The third name given to sillimanite was bucholzite, chosen by German scientist Dr. Rudolph Brandes when he described the mineral in 1819. The name honored Brandes’ mentor, renowned German chemist Christian Friedrich Bucholz. 

The specimens Brandes described came from the Tyrolese Alps. However, the description was limited, describing the mineral as grayish-white with a slight yellow undertone. 

In 1824, American scientist George Thomas Bowen studied specimens from Connecticut, USA, and gave sillimanite its current name to honor American geologist and chemist Benjamin Silliman, Sr. Silliman was also the first professor at Yale University to educate on mineralogy.

Despite its many names and descriptions, sillimanite remained widely unknown — that is, until the 1990s, when a relatively abundant deposit was discovered in Orissa, India. 

Sillimanite Healing Properties

Crystals like sillimanite can possess certain abilities as healing stones. White sillimanite can offer cleansing and spiritual awareness, like other white stones. On the other hand, green sillimanite shares green gemstone benefits like optimism and growth.

What other physical and emotional benefits does sillimanite have in store?

Physical Healing

Physically, cat’s eye sillimanite is believed to help treat eye-related issues. Sillimanite itself is said to help with:

• Heart issues (e.g. hemorrhages, blood circulation)

• Digestive problems

• Lung issues (e.g. allergies, asthma)

• Aging

• Endorphin production

Emotional Healing

The endorphin production mentioned above plays into sillimanite’s reputation as a happiness stone. Its properties are especially helpful if you feel despondent or “in a rut,” as it’s said to ease frustration and help you find what excites you again.

Additionally, sillimanite crystals are popularly used for facilitating easier decision-making, clearing negativity, and harmonizing emotions.

Chakra Healing

Chakra healing involves resolving negative symptoms associated with a blocked chakra, or energy center. Crystals are often used for specific chakras based on their coloring. 

As a chakra stone, sillimanite can open any of the chakras (energy centers), but it’s especially helpful for balancing the heart chakra — bringing in love and acceptance. Meanwhile, black or brown fibrolite opens the root chakra for restored security and emotional connection.

Sillimanite Gemstone Properties

Experts determine the objective value of sillimanite gemstones based on their color, cut, clarity, transparency, and carat weight. 

Color

Transparent to translucent sillimanite crystals can be colorless, white, yellow, green, or blue. Fibrolite (cat’s eye sillimanite) can be gray, brown, purple, blue, or yellow, along with the tones mentioned in the previous Types of Sillimanite section.

Pure sillimanite is colorless, so other hues usually come from impurities or inclusions. Blue or violet hues occur through a charge transfer from iron to titanium. Yellow and green hues come from iron or chromium, and brown coloring also comes from iron.

Stones with rare hues (like violet-blue, blue, or colorless) and medium to high saturation command the highest prices. 

Cut

Many gemstones present difficulties for cutting, but sillimanite’s perfect cleavage and brittle tenacity (in crystal form) make it among the trickiest gems to facet. 

Skilled gem cutters who take on the challenge often choose oval, cushion, or fancy faceted cuts. The end result is a rare and valuable treasure.

Compact sillimanite is often unfaceted. Cat’s eye stones must be cut as cabochons, but other material may become beads or carvings. 

Clarity & Transparency

Andalusite stones generally have a Type II colored gemstone clarity grade, meaning some visible inclusions are expected. In most sillimanite gems, inclusions like needle-like crystals will lower the value. One exception is cat’s eye sillimanite.

Cat’s eye sillimanite gets its chatoyancy from parallel ilmenite, hypersthene or rutile inclusions. When ilmenite is responsible, the fibrolite stones are usually blackish.

Most sillimanite crystals are translucent. Transparent sillimanite is rare and thus, more valuable. Most cat’s eye stones are opaque, so the rare translucent ones are also more valuable. 

Carat Weight

Faceted sillimanite gemstones aren’t only rare, but also typically under 5 carats. Cat’s eye stones can generally reach 10 carats, though the largest example is 35 carats.

Size may also depend on the source. Myanmar and Kenya produce sillimanite of similar quality, but Kenya’s stones are slightly smaller.

Sillimanite Formation & Sources

Like kyanite and andalusite, sillimanite forms as a secondary mineral inside sedimentary rocks undergoing metamorphosis via heat and pressure. These conditions alter both the rock and minerals inside, creating new minerals like sillimanite.

Which of the three stones will form depends on the heat and pressure levels. Sillimanite requires high temperatures, while kyanite requires high pressure. Andalusite forms under low pressure and temperatures. 

What type of rock is sillimanite found in? It’s usually found in high-grade metamorphic rocks like schists, gneiss, granites, or aluminum-rich pelites. 

With that in mind, where is sillimanite found?

Mining Locations

The best sources for sillimanite are Myanmar, Sri Lanka, and Kenya. Myanmar is known for its cat’s eye sillimanite (or “Burmese fibrolite”), while Sri Lanka produces cat’s eye sillimanite with grayish-green coloring. 

Both Sri Lanka and Myanmar are renowned for their green, blue, or violet-blue facetable crystals. Kenya produces light blue or colorless facetable crystals with similar quality to Myanmar’s stones, but generally smaller.

Fascinatingly, some of the largest sillimanite crystals have been found in Antarctica!

Other sillimanite sources include:

• Australia

• Austria

• Brazil

• Canada

• China

• Czechia

• France

• Germany

• India

• Italy

• Madagascar

• Norway

• Slovakia

• South Africa

• Sweden

• Tanzania

• UK (Ireland & Scotland)

• USA (notably Delaware, Idaho & Connecticut)

Of course, you’ll want to know what prices to expect, right?