RocksdayDumortierite vs Glaucophane
The quickest way to tell Dumortierite from Glaucophane: Dumortierite is clearly harder: Mohs 7–8.5 versus 6–6.5.
A denim-blue aluminum borosilicate, usually as fibrous masses or embedded in quartz.
Photo: Géry PARENT, CC BY-SA 3.0
The blue amphibole that colors blueschist, formed when ocean crust is dragged to depth and stays cool.
Photo: Didier Descouens, CC BY-SA 4.0
Quick tests
- Hardness: Dumortierite is clearly harder: Mohs 7–8.5 versus 6–6.5.
- Streak (rub on unglazed porcelain): Dumortierite leaves a white streak; Glaucophane leaves gray.
Key differences
- Dumortierite is harder and usually massive blue quartz, without amphibole cleavage.
Side by side
| Property | Dumortierite | Glaucophane |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 7–8.5 | 6–6.5 |
| Luster | Vitreous to dull | Vitreous to silky |
| Streak | White | Gray |
| Colors | Blue, Purple | Blue, Gray |
| Crystal system | Orthorhombic | Monoclinic |
| Specific gravity | 3.3–3.4 | 3–3.2 |
| Formula | Al₇BO₃(SiO₄)₃O₃ | Na₂(Mg₃Al₂)Si₈O₂₂(OH)₂ |
| Found in | Madagascar; Brazil | Île de Groix, France; Franciscan Complex, California |
Properties that are the same for both are grayed out.
How to identify dumortierite
- Denim blue to violet, opaque and fibrous.
- Very hard and tough.
- Often speckled through quartz.
Crystal system: Orthorhombic
Full Dumortierite guide →How to identify glaucophane
- Blue blades or fibrous patches in a tough metamorphic rock.
- Two amphibole cleavages.
- Hardness about 6.
Crystal system: Monoclinic
Full Glaucophane guide →More comparisons
Frequently asked questions
What is the difference between dumortierite and glaucophane?
Dumortierite is a mineral with a Mohs hardness of 7–8.5 and a vitreous to dull luster, usually blue or purple. Glaucophane is a mineral with a Mohs hardness of 6–6.5 and a vitreous to silky luster, usually blue or gray.
How can you tell dumortierite from glaucophane?
Dumortierite is clearly harder: Mohs 7–8.5 versus 6–6.5. Dumortierite leaves a white streak; Glaucophane leaves gray. Dumortierite is harder and usually massive blue quartz, without amphibole cleavage.
Could you tell them apart from a blurry close-up?
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