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, 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.