Identification guide · Silicate Minerals
How to Identify Sillimanite
Sillimanite is usually white, gray, and brown with a vitreous luster and is transparent. It has a Mohs hardness of 7. It scratches glass and a steel knife and cannot be scratched by a steel file. Its streak is white. It is most often confused with Andalusite, Quartz, and Garnet; the tests below separate them.
Sillimanite identification checklist
| Color | White, Gray, Brown, Pale green, Yellow | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Vitreous | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Transparent | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 7 | It scratches glass and a steel knife and cannot be scratched by a steel file. |
| Streak | White | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Perfect on {010} | |
| Fracture | Uneven | |
| Specific gravity | 3.24 | Heavier than it looks (specific gravity about 3.24). |
| Crystal system | Orthorhombic |
Sillimanite look-alikes and how to tell them apart
Both sit near Mohs 7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Sillimanite vs Andalusite →
Both sit near Mohs 7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Sillimanite vs Quartz →
Both sit near Mohs 7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Sillimanite vs Garnet →
Sillimanite is harder (Mohs 7) than Scapolite (Mohs 6), so Sillimanite will scratch Scapolite and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Sillimanite vs Scapolite →
Field tips
- Look for slender prismatic crystals or fibrous masses with one perfect cleavage direction.
- Sillimanite is hard, about seven, helping separate it from softer fibrous minerals.
- It can resemble andalusite and kyanite; crystal habit and locality help distinguish them.
Sillimanite identification FAQ
How can you tell if a rock is Sillimanite?
Check the color (white, gray, brown, and pale green), the vitreous luster, a Mohs hardness of 7 and a white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Sillimanite?
Sillimanite has a Mohs hardness of 7. It scratches glass and a steel knife and cannot be scratched by a steel file.
What is the difference between Sillimanite and Andalusite?
Sillimanite (Mohs 7, vitreous luster) and Andalusite (Mohs 6.5–7.5, vitreous luster) look alike, but hardness, streak and heft usually separate them. See the full Sillimanite vs Andalusite comparison for a side-by-side table.
Is Sillimanite worth anything?
Entry-level Sillimanite specimens sell for around $10, typical collector pieces about $70, and premium examples $450 or more depending on size, quality and locality.
What color is Sillimanite?
Sillimanite is most often white, gray, brown, and pale green. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
