Osumilite is a rare cyclosilicate mineral that typically forms as small, hexagonal prismatic crystals in volcanic or high-grade metamorphic environments. Collectors value it for its deep blue to blue-violet coloration, which can closely resemble cordierite or beryl upon initial inspection.
Is this osumilite?
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 osumilite with a known reference. Osumilite sits at Mohs 5-5.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: blue-gray.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: black, dark blue, dark brown, pink, gray.
- 5Look at form & habitCrystal system: hexagonal. Typical habit: prismatic crystals.
Often confused with
Osumilite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Cordierite is the harder of the two (Mohs 7-7.5 vs. 5-5.5); streak differs, so Osumilite leaves blue-gray, Cordierite leaves white.
How to tell apart: Beryl is the harder of the two (Mohs 7.5-8 vs. 5-5.5); streak differs, so Osumilite leaves blue-gray, Beryl leaves white.
Often found alongside osumilite
Minerals reported to co-occur with osumilite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KFe₂(Al₅Si₁₀)O₃₀
- Mohs hardness
- 5-5.5
- Density
- 2.58-2.68 g/cm³
- Streak
- Blue-gray
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Hexagonal
- Crystal habit
- Prismatic crystals
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector, Mineralogical research
- Host rock
- In cavities and the groundmass of rhyolite and dacite, and in high-grade contact metamorphic rocks and xenoliths
Where rockhounds find osumilite
Classic worldwide localities
- Obsidian Cliffs, Lane Co., Oregon, USA
- Sakkabira and Hayato, Kagoshima Prefecture, and from the Asama volcano, northwest of Tokyo, Yamanashi Prefecture, Japan
- Funtanifigu quarry, Monte Arci, near Oristano, Sardinia, Italy
- Bellerberg and Nickenicher Sattel volcanos, Eifel district, Germany
Field-hunting tip
Where it forms: In cavities and the groundmass of rhyolite and dacite, and in high-grade contact metamorphic rocks and xenoliths. If you start seeing tridymite, cristobalite, quartz in float, you are in the right ground. Typical habit: prismatic crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify osumilite?+
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What rocks look like osumilite?+
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Find osumilite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
