Dumortierite is a beautiful fibrous mineral best known for its deep denim-blue or violet coloration. It is often found as dense, massive aggregates in metamorphic rocks, making it a favorite for lapidary work and cabochon cutting.
Is this dumortierite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch dumortierite with a known reference. Dumortierite sits at Mohs 7-8.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Dumortierite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Dumortierite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: blue, violet-blue, reddish-brown, greenish-blue.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: fibrous, massive, or as acicular prismatic crystals.
Often confused with
Dumortierite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Dumortierite is noticeably harder (Mohs 7-8.5 vs. 5.5-6).
How to tell apart: Dumortierite is noticeably harder (Mohs 7-8.5 vs. 5-5.5); streak differs, so Dumortierite leaves white, Lazurite leaves light blue.
Often found alongside dumortierite
Minerals reported to co-occur with dumortierite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Al₇(BO₃)(SiO₄)₃O₃
- Mohs hardness
- 7-8.5
- Density
- 3.3-3.4 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Fibrous, Massive, Or as Acicular Prismatic Crystals
- Cleavage
- Distinct in One Direction
- Rarity
- Uncommon
- Uses
- Collector, Lapidary, Decorative
- Host rock
- Aluminum-rich Metamorphic Rocks, Pegmatites
- Typical price
- $5-50 for small specimens, $100+ for high-quality clusters or carvings
Where rockhounds find dumortierite
3 mapped spotsClassic worldwide localities
- France
- Namibia
- USA (Nevada)
- Brazil
- Madagascar
Field-hunting tip
Look in aluminum-rich metamorphic rocks, pegmatites country, the host setting where dumortierite typically forms. If you start seeing quartz, muscovite, kyanite in float, you are in the right ground. Field specimens usually show a fibrous, massive, or as acicular prismatic crystals habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in New Mexico, Montana. Start trip planning there.
Common questions
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Find dumortierite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
