Petoskey Stone
Petoskey Stone is fossil (rugose colonial coral), CaCO₃ (calcite). In the hand, it rates 3 to 4 on the Mohs scale, and it runs gray and tan with darker hexagon 'eyes'. Gray pebble covered in tight hexagonal 'sunbursts', each six-sided cell a fossil coral mouth with radiating rays, popping vividly when wet or polished.
The geology.
Petoskey Stone carries the formula CaCO₃ (calcite). It belongs to the, system. Catalogues list it as fossil (rugose colonial coral).
It sits at 3 to 4 on the Mohs scale. The surface reads dull dry; waxy polished. Light behaviour is opaque. Specific gravity runs 2.6 to 2.7. The streak is white. It breaks with uneven fracture.
Gray pebble covered in tight hexagonal 'sunbursts', each six-sided cell a fossil coral mouth with radiating rays, popping vividly when wet or polished.
It reaches buyers as beach pebbles, polished stones, carvings or jewelry.
The origins.
Devonian reef rock of Lake Michigan shores, Petoskey, Michigan (USA) and surrounding beaches.
On value, the database notes: affordable; large flawless polished stones are regional treasures.
Traditional associations.
In energy work traditions, associated with emotional clarity and ancestral connection.
We keep this section because people ask. It sits below the geology on purpose, and none of it describes something the stone does.
Spotting the real thing.
Gray pebble covered in tight hexagonal 'sunbursts', each six-sided cell a fossil coral mouth with radiating rays, popping vividly when wet or polished.
Resin replicas exist in tourist shops, real stones are cold, fizz in vinegar, and no two patterns repeat.
| Looks like | How to tell them apart |
|---|---|
| Charlevoix stone | Its sibling fossil coral has smaller plain cells without the dark radiating 'pupils' |
| Fossil honeycomb coral (favosite) | Favosites' tubes are tiny and lack the eye-with-lashes pattern |
| Concrete with aggregate | Concrete shows random gravel, not repeating hexagons |
At-home tests.
Before anyone talks you into a lab report, these are the checks worth doing yourself on petoskey stone.
| Test | What you should see |
|---|---|
| By eye | Hexagonal cells with radiating lines |
| Acid check | Fizzes with vinegar (calcite) |
| By eye | Pattern leaps out when wetted |
Destructive tests belong to a gemologist, not a kitchen table. If a piece matters to you, pay for an appraisal before you take a file to it.
Care & handling.
At 3 to 4 on the Mohs scale this is a soft material. Keep it as a display piece, store it apart from harder stones, and skip daily-wear settings.
It breaks with uneven fracture, which is worth knowing before you set it or drop it.
Clean it by hand, dry it properly, and keep it away from ultrasonic cleaners and hot steam.
A deeper look.
Extended detail on composition, sourcing, authentication and market value.
The physical record
The formula on record is CaCO₃ (calcite). Crystal system:,. Specific gravity: 2.6 to 2.7. Streak: White. Cleavage and fracture: Uneven fracture.
How it reaches the market
Devonian reef rock of Lake Michigan shores, Petoskey, Michigan (USA) and surrounding beaches.
What it gets confused with
Charlevoix stone: Its sibling fossil coral has smaller plain cells without the dark radiating 'pupils'. Fossil honeycomb coral (favosite): Favosites' tubes are tiny and lack the eye-with-lashes pattern. Concrete with aggregate: Concrete shows random gravel, not repeating hexagons.
What it is worth
Affordable; large flawless polished stones are regional treasures.
Where buyers get caught
Resin replicas exist in tourist shops, real stones are cold, fizz in vinegar, and no two patterns repeat.
One more thing
Michigan's state stone is a 350-million-year-old coral reef, beachcombers hunt it each spring as ice heaves fresh stones ashore.
An education-first guide.
We do not stock petoskey stone. Writing it up regardless is the point of a reference library.
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