Comparison · Minerals
Chiastolite vs Onyx
Chiastolite and Onyx are easy to confuse because both are minerals. Both sit near Mohs 6.5–7.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Below is a side-by-side comparison of their properties and the quick tests that separate them.
Key differences between Chiastolite and Onyx
- 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 feels noticeably heavier for its size (specific gravity 3.13–3.21 versus 2.6–2.65 for Onyx).
- They overlap in brown and white, but Chiastolite also comes in gray and Onyx also comes in black, multicolored, and red.
- Well-formed crystals give it away: Chiastolite is Orthorhombic, Onyx is Trigonal (cryptocrystalline).
Chiastolite vs Onyx properties
| Property | Chiastolite | Onyx |
|---|---|---|
| Type | Minerals | Minerals |
| Mohs hardness | 6.5–7.5 | 6.5–7 |
| Luster | Vitreous | Waxy to vitreous |
| Streak | White | White |
| Transparency | Translucent to opaque | Translucent to opaque |
| Specific gravity | 3.13–3.21 | 2.6–2.65 |
| Crystal system | Orthorhombic | Trigonal (cryptocrystalline) |
| Common colors | Gray, Brown, White with dark cross-shaped carbon inclusions | Black and white banded, Brown and white banded, Red and white banded |
| Rarity | common | common |
| Typical specimen value | $2–$150+ | $2–$150+ |
Which is more valuable?
Typical specimens of Chiastolite and Onyx trade in a similar range, around $20.
About Chiastolite
Chiastolite is a variety of the aluminosilicate mineral andalusite distinguished by a dark, cross-shaped pattern of carbonaceous inclusions visible in cross-section. The cross forms because carbon and clay impurities present in the original rock get trapped along specific growth directions as the andalusite crystal expands through the surrounding slate or hornfels. It has long been collected and worn as a natural talisman precisely because of this striking symmetrical pattern.
About Onyx
Onyx is a banded variety of chalcedony, a cryptocrystalline form of quartz, distinguished from agate by its straight, parallel bands rather than agate's characteristically curved layers. Most commercial onyx sold in black and white banded slabs is actually dyed to enhance contrast, since naturally occurring true onyx with strong black-and-white banding is relatively uncommon. Chemically identical to other quartz varieties, onyx owes its distinctive look entirely to microscopic banding formed during slow silica deposition.
Chiastolite vs Onyx FAQ
Is Chiastolite the same as Onyx?
No. Both are minerals, but they are distinct species with different hardness, streak or luster. Both sit near Mohs 6.5–7.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
How can you tell Chiastolite from Onyx?
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 feels noticeably heavier for its size (specific gravity 3.13–3.21 versus 2.6–2.65 for Onyx). They overlap in brown and white, but Chiastolite also comes in gray and Onyx also comes in black, multicolored, and red.
Which is harder, Chiastolite or Onyx?
They are about equally hard, both near Mohs 6.5–7.5.
Which is more valuable, Chiastolite or Onyx?
Typical specimens of Chiastolite and Onyx trade in a similar range, around $20.
