Petoskey Stone
A variety of Coral
What is Petoskey Stone?
Petoskey Stone can be found in only one location in the world and that’s along the Great Lake’s shores of Michigan’s lower peninsula. In 1965, this stone unique to Michigan (USA) became its official state stone. Some 350 million years ago, these fossil rocks were living colonies of corals named “rugose”.
Optical Properties
- Refractive Index
- 1.486-1.658
- Birefringence
- 0.160-0.172
- Pleochroism
- None
- Optical Character
- Uniaxial negative
Market Value Factors
Pricing varies for every rock and mineral, so use these universal factors to gauge Petoskey Stone before comparing listings or appraisals.
Size & Weight
Larger, intact specimens usually command higher prices.
Rarity & Demand
Scarce material or popular varieties sell at a premium.
Condition & Finish
Chips, repairs, and heavy wear lower value; clean prep helps.
Treatment & Provenance
Untreated specimens with documented locality are prized.
Where to Find Petoskey Stone
RockHoundR, the free-to-download rockhounding map from the Ruby Glint team, shows where collectors find petoskey stone in the US, with land-access notes for BLM land and national forests. Collecting is banned in national parks.
See where to find petoskey stoneAlways check the local rules and get the landowner's permission before you collect.
Key Characteristics
Composition of Petoskey Stone
Calcite
More Images
Quick Facts
Physical Properties
- Hardness (Mohs)
- 3
- Rock tumbling
- Not recommended
- Density
- 2.3 - 2.7 g/cm³
- Crystal System
- Trigonal
Chemical Properties
- Chemical Formula
- CaCO3
Also Known As
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Petoskey Stone FAQs
How do I identify Petoskey Stone?
Petoskey Stone can be identified by its hardness of 3 on the Mohs scale, Trigonal crystal system. Look for these key characteristics when examining specimens.
How hard is Petoskey Stone?
Petoskey Stone has a hardness of 3 on the Mohs scale. This makes it a soft mineral that can be scratched easily.
What is the composition of petoskey stone of Petoskey Stone?
Calcite