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Sillimanite
Sillimanite is nesosilicates (Al2SiO5 polymorph), Al2SiO5. In the hand, it rates 6.5 to 7.5 on the Mohs scale, and it runs grey-white and pale blue. Silky fibrous bundles or slender glassy prisms in high-grade metamorphic rock, the high-temperature sibling of kyanite and andalusite, often cut as a chatoyant cat's-eye.
The geology.
Sillimanite carries the formula Al2SiO5. It belongs to the orthorhombic system. Its listing category is nesosilicates (Al2SiO5 polymorph).
It sits at 6.5 to 7.5 on the Mohs scale. The surface reads vitreous to silky. Light behaviour is transparent to translucent. Specific gravity runs 3.23 to 3.27. The streak is white. It breaks with perfect cleavage one direction; splintery fracture.
Silky fibrous bundles or slender glassy prisms in high-grade metamorphic rock, the high-temperature sibling of kyanite and andalusite, often cut as a chatoyant cat's-eye.
It turns up mostly as fibrous mats (fibrolite), slender prisms in gneiss, cat's-eye cabochons or faceted rarities.
The origins.
High-temperature metamorphic rocks, India and Sri Lanka gem gravels, Brazil, and classic American gneiss terrains.
On value, the database notes: affordable; clean faceted stones and sharp cat's-eyes are collector items.
Traditional associations.
In energy work traditions, sillimanite is said to align scattered efforts into one clear line of intent.
This is lore, and we label it as lore. The measurable facts are further up the page.
Spotting the real thing.
Silky fibrous bundles or slender glassy prisms in high-grade metamorphic rock, the high-temperature sibling of kyanite and andalusite, often cut as a chatoyant cat's-eye.
Cat's-eye sillimanite is sometimes sold as rarer cat's-eye chrysoberyl, chrysoberyl is notably harder (8.5) and denser.
| Looks like | How to tell them apart |
|---|---|
| Kyanite | Same chemistry; kyanite forms blue blades with split hardness (soft along, hard across) |
| Andalusite | Same chemistry again; andalusite forms stubby square prisms, sometimes with chiastolite crosses |
| Tremolite/actinolite fibers | Amphiboles are softer with two cleavages at 120° |
At-home tests.
A loupe and a window seat settle most questions about sillimanite. No workshop required.
| Test | What you should see |
|---|---|
| Scratch check | Hardness ~7 in all directions (unlike kyanite) |
| By eye | Splintery fibrous habit with silky sheen |
| By eye | Found in gneiss/schist of the highest metamorphic grade |
Anything that scratches, burns or dissolves the piece is a lab job. Get it appraised rather than testing it apart.
Care & handling.
At 6.5 to 7.5 on the Mohs scale it handles careful wear but scratches against quartz and dust. Store it separately and take rings off before rough work.
It breaks with perfect cleavage one direction; splintery fracture, which is worth knowing before you set it or drop it.
Warm water and a soft cloth clean almost everything. Ultrasonic tanks, steamers and household cleaners are where pieces get ruined.
Fibrous fibrolite dust is a respiratory irritant, avoid grinding without protection; solid stones are fine to handle.
A deeper look.
Extended detail on composition, sourcing, authentication and market value.
The physical record
The formula on record is Al2SiO5. Crystal system: Orthorhombic. Specific gravity: 3.23 to 3.27. Streak: White. Cleavage and fracture: Perfect cleavage one direction; splintery fracture.
How it reaches the market
High-temperature metamorphic rocks, India and Sri Lanka gem gravels, Brazil, and classic American gneiss terrains.
What it gets confused with
Kyanite: Same chemistry; kyanite forms blue blades with split hardness (soft along, hard across). Andalusite: Same chemistry again; andalusite forms stubby square prisms, sometimes with chiastolite crosses. Tremolite/actinolite fibers: Amphiboles are softer with two cleavages at 120°.
What it is worth
Affordable; clean faceted stones and sharp cat's-eyes are collector items.
Where buyers get caught
Cat's-eye sillimanite is sometimes sold as rarer cat's-eye chrysoberyl, chrysoberyl is notably harder (8.5) and denser.
One more thing
Sillimanite is Delaware's state mineral and a geologist's thermometer, kyanite, andalusite, and sillimanite are the same formula frozen at different pressure-temperature depths, so finding one tells you how hard the rock was cooked.
An education-first guide.
No product links here. We do not sell sillimanite, and a guide that only covers our own shelves would not be much of a guide.
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