Grandidierite is a rare magnesium aluminum borosilicate mineral highly prized by collectors for its distinct blue-green to greenish-blue color. It most frequently occurs as prismatic crystals or granular aggregates in metamorphic environments and is notably rare in gem-quality, faceted material.
Is this grandidierite?
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 grandidierite with a known reference. Grandidierite sits at Mohs 7.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Grandidierite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Grandidierite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: blue-green, bluish-green, green.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: prismatic crystals, granular, massive.
Often confused with
Grandidierite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside grandidierite
Minerals reported to co-occur with grandidierite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Mg,Fe)Al₃(BSiO₉)O₂
- Mohs hardness
- 7.5
- Density
- 2.98-3.00 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Prismatic Crystals, Granular, Massive
- Cleavage
- Perfect On {100}, Good On {010}
- Rarity
- Rare
- Uses
- Gemstone, Collector
- Host rock
- Aluminum-rich Metamorphic Rocks, Pegmatites
- Typical price
- $50-500 per carat for gem quality, high for faceted specimens
Where rockhounds find grandidierite
Classic worldwide localities
- Madagascar
- Sri Lanka
- Norway
- Namibia
- United States
Field-hunting tip
Look in aluminum-rich metamorphic rocks, pegmatites country — that is the host setting where grandidierite typically forms. If you start seeing quartz, k-feldspar, garnet in float, you are in the right ground. Field specimens usually show a prismatic crystals, granular, massive habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find grandidierite on the map
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