Identification guide · Minerals
How to Identify Dumortierite
Dumortierite ranges from violet-blue to deep blue, occasionally brown or pink, typically occurring as dense fibrous masses rather than large distinct crystals, with a vitreous to silky luster depending on fiber orientation. Its streak is white to pale blue, hardness runs about 7 to 8.5, notably harder than most blue minerals it resembles, and it shows no obvious cleavage, breaking instead with a splintery fracture. It is frequently confused with sodalite and lazulite, but dumortierite's higher hardness and fibrous internal structure distinguish it under a hand lens.
Dumortierite identification checklist
| Color | Blue, Violet-blue, Brownish, Pink (rare) | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Vitreous, silky when fibrous | 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 | 7–8.5 | It scratches glass, steel and even quartz; only harder gems will mark it. |
| Streak | White | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Distinct on {100} | |
| Fracture | Uneven, splintery | |
| Specific gravity | 3.3–3.4 | Heavier than it looks (specific gravity about 3.3499999999999996). |
| Crystal system | Orthorhombic |
Dumortierite look-alikes and how to tell them apart
Both sit near Mohs 7–8.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Dumortierite vs Quartz →
Dumortierite is harder (Mohs 7–8.5) than Calcite (Mohs 3), so Dumortierite will scratch Calcite and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Dumortierite vs Calcite →
Dumortierite is harder (Mohs 7–8.5) than Axinite (Mohs 6.5–7), so Dumortierite will scratch Axinite and not the other way round. It scratches glass and a steel knife and cannot be scratched by a steel file. Dumortierite vs Axinite →
Dumortierite is harder (Mohs 7–8.5) than Diaspore (Mohs 6.5–7), so Dumortierite will scratch Diaspore and not the other way round. It scratches glass and a steel knife and cannot be scratched by a steel file.
Dumortierite identification FAQ
How can you tell if a rock is Dumortierite?
Check the color (blue, violet-blue, brownish, and pink (rare)), the vitreous, silky when fibrous luster, a Mohs hardness of 7–8.5 and a white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Dumortierite?
Dumortierite has a Mohs hardness of 7–8.5. It scratches glass, steel and even quartz; only harder gems will mark it.
What is the difference between Dumortierite and Quartz?
Dumortierite (Mohs 7–8.5, vitreous, silky when fibrous luster) and Quartz (Mohs 7, vitreous, greasy on fracture surfaces luster) look alike, but hardness, streak and heft usually separate them. See the full Dumortierite vs Quartz comparison for a side-by-side table.
Is Dumortierite worth anything?
Entry-level Dumortierite 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 Dumortierite?
Dumortierite is most often blue, violet-blue, brownish, and pink (rare). Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
