Seamanite is a very rare manganese phosphate-borate mineral primarily found in the iron mines of Michigan. It typically occurs as small, delicate, yellow to brownish prismatic crystals associated with other manganese minerals.

Hardness

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralrare

Seamanite

Hardness: 4
Luster: Vitreous
Streak: White
Transparency: Transparent

Is this seamanite?

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 seamanite with a known reference. Seamanite sits at Mohs 4, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Seamanite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Seamanite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: yellow, yellow-brown, brown.
5
Look at form & habit
Crystal system: orthorhombic. Typical habit: prismatic crystals.

Often confused with

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

mineral

Kurnakovite

Mohs 3

How to tell apart: Seamanite is noticeably harder (Mohs 4 vs. 3).

How to tell apart: Luster reads vitreous on Seamanite and silky on Sussexite.

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Often found alongside seamanite

Minerals reported to co-occur with seamanite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: Mn₃(PO₄)(BO₃)(OH)₂
Mohs hardness: 4
Density: 2.95 g/cm³
Colors: Yellow Yellow-brown Brown
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Orthorhombic
Crystal habit: Prismatic Crystals
Cleavage: Good in One Direction
Rarity: Rare
Uses: Collector
Host rock: Manganiferous Ore Deposits
Typical price: $50-500 depending on specimen quality and size

Where rockhounds find seamanite

Classic worldwide localities

Michigan, USA

Field-hunting tip

Look in manganiferous ore deposits country, the host setting where seamanite typically forms. If you start seeing hausmannite, barite, hematite 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 seamanite?+

Mohs hardness is 4. It typically shows a vitreous luster. The streak is white. Common colors include yellow, yellow-brown, brown.

Where is seamanite found?+

Notable localities include Michigan, USA.

How much is seamanite worth?+

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

What rocks look like seamanite?+

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

What minerals are found with seamanite?+

Seamanite commonly co-occurs with Hausmannite, Barite, Hematite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does seamanite form in?+

Seamanite typically forms in manganiferous ore deposits. Working float back to the host body is the standard way to chase a fresh occurrence.

What is seamanite used for?+

Seamanite is used in collector.

Find seamanite on the map

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