Sillimanite is a high-temperature aluminum silicate often found in regional metamorphic rocks like schist and gneiss. It is best known for its fibrous, splintery habit, which earned it the synonym 'fibrolite', though it also occurs as slender, long prismatic crystals.
Is this sillimanite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch sillimanite with a known reference. Sillimanite sits at Mohs 6.5-7.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Sillimanite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Sillimanite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, colorless, gray, brown, pale blue, yellowish.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: fibrous, acicular, prismatic crystals.
Often confused with
Sillimanite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside sillimanite
Minerals reported to co-occur with sillimanite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Al₂SiO₅
- Mohs hardness
- 6.5-7.5
- Density
- 3.23-3.27 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Fibrous, Acicular, Prismatic Crystals
- Cleavage
- Perfect Basal
- Rarity
- Common
- Uses
- Collector, Refractory Materials, Gemstone
- Host rock
- Metamorphic Rocks
- Typical price
- $10-100 per specimen depending on size and quality
Where rockhounds find sillimanite
17 mapped spotsClassic worldwide localities
- France
- USA
- India
- Sri Lanka
- Myanmar
Field-hunting tip
Look in metamorphic rocks country — that is the host setting where sillimanite typically forms. If you start seeing garnet, quartz, mica in float, you are in the right ground. Field specimens usually show a fibrous, acicular, prismatic crystals habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Georgia, South Carolina, Alabama — start trip planning there.
Common questions
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Find sillimanite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
