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.

Hardness

5-5.5

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralrareOsumilite Group family

Osumilite

Hardness: 5-5.5
Luster: Vitreous
Streak: White
Transparency: Transparent

Is this osumilite?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try 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.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Osumilite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Osumilite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: blue, blue-violet, colorless, brown.
5
Look at form & habit
Crystal system: hexagonal. Typical habit: prismatic crystals.

Often confused with

Osumilite vs. its common look-alikes — and how to tell them apart in the field.

mineral

Cordierite

Mohs 7-7.5

How to tell apart: Cordierite is the harder of the two (Mohs 7-7.5 vs. 5-5.5).

How to tell apart: Beryl is the harder of the two (Mohs 7.5-8 vs. 5-5.5).

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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: (K,Na)(Fe,Mg)₂(Al,Fe,Mg)₃(Si,Al)₁₂O₃₀
Mohs hardness: 5-5.5
Density: 2.63-2.65 g/cm³
Colors: Blue Blue-violet Colorless Brown
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Hexagonal
Crystal habit: Prismatic Crystals
Cleavage: None
Rarity: Rare
Uses: Collector, Mineralogical Research
Host rock: Volcanic Rocks, Granulite Facies Metamorphic Rocks
Typical price: $50-500 depending on crystal size and quality

Where rockhounds find osumilite

Classic worldwide localities

Osumi Islands, Japan
Eifel, Germany
Labrador, Canada
Madagascar
Italy

Field-hunting tip

Look in volcanic rocks, granulite facies metamorphic rocks country — that is the host setting where osumilite typically forms. If you start seeing tridymite, sanidine, magnetite in float, you are in the right ground. Field specimens usually show a prismatic crystals habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify osumilite?+

Mohs hardness is 5-5.5. It typically shows a vitreous luster. The streak is white. Common colors include blue, blue-violet, colorless, brown.

Where is osumilite found?+

Notable localities include Osumi Islands, Japan; Eifel, Germany; Labrador, Canada; Madagascar; Italy.

How much is osumilite worth?+

Typical asking prices fall in the range of $50-500 depending on crystal size and quality. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like osumilite?+

Osumilite is most often confused with Cordierite, Beryl. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with osumilite?+

Osumilite commonly co-occurs with Tridymite, Sanidine, Magnetite, Hypersthene. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does osumilite form in?+

Osumilite typically forms in volcanic rocks, granulite facies metamorphic rocks. Working float back to the host body is the standard way to chase a fresh occurrence.

What is osumilite used for?+

Osumilite is used in collector, mineralogical research.

Find osumilite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.