What's That Rock?Fossil coral
Also called: Petoskey stone
The stony skeleton of ancient coral colonies. Michigan's Petoskey stones are a famous example, with a six-sided honeycomb pattern.
How to identify Fossil coral
- Honeycomb or branching pattern
- Often found as rounded pebbles on beaches
- Fizzes in vinegar if calcite
Often confused with
- Honeycomb weathering patterns
- Sponge fossils
Is Fossil coral valuable?
Common.
Typical retail prices
Polished piece$5–40each
Typical retail ranges in US dollars, compiled from public dealer and marketplace listings in October 2026. Real prices vary widely with quality, size and origin. This is general information, not an appraisal.
What changes the price
- Pattern: a clear honeycomb (Petoskey) or branching form is what buyers want
- Polish: a smooth, glossy polished stone costs more than a rough fossil
- Size and origin: large, complete pieces cost more; legally collected material only
- Collecting rules: some localities restrict collecting
Uses
- Collecting and display
Care
Dissolves slowly in acid.
Identifying a stone from a description or a photo is an estimate. Many stones look alike. Ask an expert or a gem lab before you buy, sell or rely on an identification.
Updated October 2026