Osumilite-(Mg) is a rare cyclosilicate that often forms short, hexagonal prismatic crystals reminiscent of beryl. It is primarily found in high-grade metamorphic rocks and specific volcanic environments, often occurring as inclusions or tiny crystals within larger mineral matrices. Collectors prize it for its rarity and its clear, sometimes pale-blue crystalline appearance.
Is this osumilite-(mg)?
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-(mg) with a known reference. Osumilite-(Mg) sits at Mohs 5-6, 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: pearly.
- 4Match the color rangeCompare against the expected color range: brown, green, dark blue, black.
- 5Look at form & habitCrystal system: hexagonal. Typical habit: prismatic crystals, anhedral grains.
Often confused with
Osumilite-(Mg) vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Beryl is the harder of the two (Mohs 7.5-8 vs. 5-6); luster reads pearly on Osumilite-(Mg) and vitreous, resinous on Beryl.
How to tell apart: Cordierite is the harder of the two (Mohs 7-7.5 vs. 5-6); luster reads pearly on Osumilite-(Mg) and vitreous on Cordierite.
How to tell apart: Luster reads pearly on Osumilite-(Mg) and vitreous on Milarite.
Often found alongside osumilite-(mg)
Minerals reported to co-occur with osumilite-(mg). Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KMg₂(Al₅Si₁₀)O₃₀
- Mohs hardness
- 5-6
- Density
- 2.58-2.68 g/cm³
- Streak
- White
- Luster
- Pearly
- Transparency
- Semitransparent
- Crystal system
- Hexagonal
- Crystal habit
- Prismatic crystals, anhedral grains
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In fused xenoliths and contact metamorphic rocks
Where rockhounds find osumilite-(mg)
Classic worldwide localities
- Tieveragh, Co. Antrim, Ireland
- Vikeså, Rogaland, Norway
- Iriki, Kagoshima Prefecture, Japan
- Haneyama, Oita Prefecture, Japan
- Akagi volcano, Gumma Prefecture, Japan
- Rishiri Island, Japan
Field-hunting tip
Where it forms: In fused xenoliths and contact metamorphic rocks. If you start seeing cordierite, sodic plagioclase, sanidine in float, you are in the right ground. Typical habit: prismatic crystals, anhedral grains. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify osumilite-(mg)?+
Where is osumilite-(mg) found?+
What rocks look like osumilite-(mg)?+
What minerals are found with osumilite-(mg)?+
What kind of rock does osumilite-(mg) form in?+
What is osumilite-(mg) used for?+
Find osumilite-(mg) on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
