Dumortierite Identification Guide
Identify dumortierite, the blue-violet aluminum borosilicate, by its fibrous masses, strong color, and the tests that separate it from sodalite, lazulite, and lapis.
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What It Looks Like
Dumortierite is an aluminum borosilicate that occurs most often as dense, fibrous to columnar masses and felted aggregates rather than visible crystals. Color is its calling card: deep blue, violet-blue, lavender, and sometimes pinkish or brownish. Luster is vitreous to dull (silky in fibrous masses). It is translucent to opaque, with strong pleochroism in transparent material. Massive dumortierite is often carved or tumbled and can be mistaken for sodalite or lapis.
Telltale Visual Cues
- Intense, even blue to violet color, often with a slightly fibrous or streaky texture.
- Silky sheen on fibrous aggregates.
- Hardness clearly greater than a knife — it scratches glass easily.
Step-by-Step Field-ID Checklist
- Note the color: rich blue to violet, frequently uniform across the piece.
- Look for fibrous texture: parallel or radiating fibers and a silky luster are typical.
- Test hardness: dumortierite is Mohs 7–8.5 and scratches glass and quartz readily — much harder than sodalite or lapis.
- Check streak: white to pale blue.
- Look for pleochroism in transparent grains: blue, violet, and reddish tones when rotated.
- Confirm no obvious cleavage in massive material; fracture is uneven.
Key Diagnostic Tests
- Mohs hardness: 7–8.5 — the strongest distinguishing trait against blue look-alikes.
- Streak: white to very pale blue.
- Cleavage: good in one direction in crystals, but rarely seen in massive material.
- Density: ~3.3–3.4 g/cm³, notably heavier than quartz.
- Acid: inert to dilute HCl.
- Pleochroism: strong, a useful confirmation in clear fragments.
Common Look-Alikes
- Sodalite: blue but much softer (Mohs 5.5–6, scratched by a knife) and lighter (~2.3); often has white veining.
- Lapis Lazuli: deep blue with pyrite flecks and calcite; softer (5–5.5) and fizzes only on calcite portions.
- Lazulite: blue and hard (5.5–6) but less than dumortierite and forms sharper crystals.
- Azurite: deep blue but soft (3.5–4) and fizzes in acid.
- Dumortierite quartz: quartz colored by dumortierite inclusions — looks similar but is dominated by quartz and is the same hardness (~7), distinguished by texture and bulk composition.
Where It Is Found
Dumortierite forms in aluminum-rich metamorphic and pegmatitic rocks, often in gneisses, schists, and quartzites. Notable sources include Brazil, Madagascar, Namibia, France (the type locality near Beaunan, Rhône), Norway, Sri Lanka, and the USA (Nevada, Arizona, California).
Frequently asked questions
How can you tell if a blue stone is dumortierite?
Test its hardness. Dumortierite is Mohs 7–8.5 and easily scratches glass and quartz, which immediately separates it from softer blue stones like sodalite, lapis, and azurite. A fibrous, silky texture and strong blue-violet color also help.
What is the difference between dumortierite and sodalite?
Both are blue, but dumortierite is much harder (7–8.5 vs sodalite's 5.5–6) and denser. Sodalite is scratched by a steel knife and often shows white veining; dumortierite will not be scratched by steel.
Is dumortierite the same as dumortierite quartz?
No. Dumortierite is the mineral itself, while dumortierite quartz is quartz colored blue by tiny dumortierite inclusions. The quartz variety is mostly silica with the same ~7 hardness throughout.
What color is dumortierite?
Most commonly deep blue to violet-blue and lavender, sometimes pink or brownish. The color is usually strong and fairly even across massive specimens.
Dumortierite identified by the community
Recent Dumortierite specimens identified with Rock Identifier.
