The Petoskey stone is a fossilized rugose coral from the Devonian period, characterized by its distinct hexagonal patterns which resemble honeycomb. Collectors often find them along the shores of Lake Michigan, where they are polished smooth by glacial activity and water movement.

Hardness

3-4

Mohs

Luster

Dull to Vitreous

Streak

White

Transparency

Opaque

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Petoskey Stones

Also: Hexagonaria percarinata

Hardness: 3-4
Luster: Dull to Vitreous
Streak: White
Transparency: Opaque

Is this petoskey stones?

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 petoskey stones with a known reference. Petoskey Stones sits at Mohs 3-4 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Petoskey Stones leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Petoskey Stones typically shows a dull to vitreous luster.
4
Match the color range
Compare against the expected color range: gray, brown, tan.
5
Look at form & habit
Typical habit: massive.

Often found alongside petoskey stones

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

All properties

Chemical formula: CaCO₃
Mohs hardness: 3-4
Density: 2.6-2.7 g/cm³
Colors: Gray Brown Tan
Streak: White
Luster: Dull to Vitreous
Transparency: Opaque
Crystal habit: Massive
Cleavage: None
Rarity: Common
Uses: Lapidary, Collector, Decorative
Host rock: Limestone
Typical price: $5-50 for polished specimens

Where rockhounds find petoskey stones

6 mapped spots

Classic worldwide localities

Michigan
USA

Field-hunting tip

Look in limestone country — that is the host setting where petoskey stones typically forms. If you start seeing calcite, quartz in float, you are in the right ground. Field specimens usually show a massive habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Michigan — start trip planning there.

Browse 6 U.S. spots for petoskey stones →

Common questions

How do you identify petoskey stones?+

Mohs hardness is 3-4. It typically shows a dull to vitreous luster. The streak is white. Common colors include gray, brown, tan.

Where is petoskey stones found?+

Notable localities include Michigan; USA.

Can I find petoskey stones in the United States?+

RockHoundR maps 6 petoskey stones rockhounding spots across 1 U.S. states — the top states are Michigan.

How much is petoskey stones worth?+

Typical asking prices fall in the range of $5-50 for polished specimens. Quality, size, and provenance can move individual specimens well outside that range.

What minerals are found with petoskey stones?+

Petoskey Stones commonly co-occurs with calcite, quartz. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does petoskey stones form in?+

Petoskey Stones typically forms in limestone. Working float back to the host body is the standard way to chase a fresh occurrence.

What is petoskey stones used for?+

Petoskey Stones is used in lapidary, collector, decorative.

Find petoskey stones on the map

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