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. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous, somewhat pearly on the cleavage.
- 4Match the color rangeCompare against the expected color range: bluish green.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: tabular.
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
- MgAl₃O₂(BO₃)SiO₄
- Mohs hardness
- 7.5
- Density
- 2.98-2.99 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous, somewhat pearly on the cleavage
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Tabular
- Cleavage
- {100}, more perfect, and {010}
- Rarity
- Rare
- Uses
- Gemstone, Collector
- Host rock
- A rare accessory mineral in aluminous boron-rich rocks, thermally and high-grade regionally metamorphosed under low pressure; in pegmatites, aplites, gneisses; in xenoliths
Where rockhounds find grandidierite
Classic worldwide localities
- Johnsburg, Warren Co., New York, USA
- Andrahomana, southwest of Taolan̈aro (Fort Dauphin), Madagascar
- Marotrana, Amboasara, Madagascar
- Vohiboly, near Behara, Madagascar
- Kachebere Hills, Mchinji district, Malawi
Field-hunting tip
Where it forms: A rare accessory mineral in aluminous boron-rich rocks, thermally and high-grade regionally metamorphosed under low pressure; in pegmatites, aplites, gneisses; in xenoliths. If you start seeing quartz, potassic feldspar, plagioclase in float, you are in the right ground. Typical habit: tabular. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify grandidierite?+
Where is grandidierite found?+
What rocks look like grandidierite?+
What minerals are found with grandidierite?+
What kind of rock does grandidierite form in?+
What is grandidierite used for?+
Find grandidierite on the map
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