Identification guide · Minerals
How to Identify Chiastolite
Chiastolite appears as elongated, squarish prisms of grayish, brownish, or pinkish andalusite with a dark cross of carbon inclusions visible when the crystal is sliced crosswise. It has a vitreous to dull luster, a white streak, and a hardness of 6.5 to 7.5, too hard for a steel knife blade to scratch. Cleavage is distinct in two directions and fracture is uneven. The telltale cross, absent in ordinary andalusite, is what separates chiastolite from its parent mineral and from superficially similar staurolite twins.
Chiastolite identification checklist
| Color | Gray, Brown, White with dark cross-shaped carbon inclusions | 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 | Translucent to opaque | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 6.5–7.5 | 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 | Good prismatic, two directions (~89°) | |
| Fracture | Uneven to splintery | |
| Specific gravity | 3.13–3.21 | Heavier than it looks (specific gravity about 3.17). |
| Crystal system | Orthorhombic |
Chiastolite look-alikes and how to tell them apart
Both sit near Mohs 6.5–7.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Chiastolite vs Chalcedony →
Both sit near Mohs 6.5–7.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Chiastolite vs Quartz →
Both sit near Mohs 6.5–7.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Chiastolite vs Onyx →
Chiastolite is harder (Mohs 6.5–7.5) than Sodalite (Mohs 5.5), so Chiastolite will scratch Sodalite 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.
Chiastolite identification FAQ
How can you tell if a rock is Chiastolite?
Check the color (gray, brown, and white with dark cross-shaped carbon inclusions), the vitreous luster, a Mohs hardness of 6.5–7.5 and a white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Chiastolite?
Chiastolite has a Mohs hardness of 6.5–7.5. It scratches glass and a steel knife and cannot be scratched by a steel file.
What is the difference between Chiastolite and Chalcedony?
Chiastolite (Mohs 6.5–7.5, vitreous luster) and Chalcedony (Mohs 6.5–7, waxy to vitreous luster) look alike, but hardness, streak and heft usually separate them. See the full Chiastolite vs Chalcedony comparison for a side-by-side table.
Is Chiastolite worth anything?
Entry-level Chiastolite specimens sell for around $2, typical collector pieces about $20, and premium examples $150 or more depending on size, quality and locality.
What color is Chiastolite?
Chiastolite is most often gray, brown, and white with dark cross-shaped carbon inclusions. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
