sedimentary (fossilized)

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

Hardness: 3.0-3.5 (Calcite based); Color: Grey, tan, or brown; Luster: Dull/earthy (unpolished) or vitreous (polished); Structure: Hexagonal corallite chambers with radiating lines (septa); Specific Gravity: 2.7

Hardness: 3
Color: Grey, tan, or brown
Luster: Dull/earthy (unpolished) or vitreous (polished)

Identified March 8, 2026More sedimentary (fossilized) →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed during the Devonian Period (approx. 350 million years ago) from ancient reef-building coral colonies; fossils were subsequently moved and smoothed by glacial action during the Pleistocene epoch.

Uses & applications

Used primarily in lapidary work for jewelry, cabochons, and decorative ornaments; also a popular specimen for fossil collectors and used in construction as landscaping stone in the Great Lakes region.

Geological facts

It is the official state stone of Michigan. The unique 'eye' pattern is only visible when the stone is wet or polished because the internal structure is made of calcite that matches the surrounding matrix.

Field identification & locations

Identify by looking for a pebble with a honeycomb-like pattern of hexagons. These are most commonly found on the beaches and in glacial till around Lake Michigan. To reveal the pattern in the field, lick or dip the stone in water.

Other sedimentary (fossilized) specimens

See all

Iridescent Ammonite

Ammonoidea (Species: likely Cleoniceras or Desmoceras); Mineral: Aragonite/Ammolite (CaCO3)

sedimentary (fossilized)

Kambaba Jasper

Stromatolite (Fossilized Algae)

sedimentary

Fossilized Shark Teeth

Carcharhiniformes or Lamniformes species (common mineral: Fluorapatite/Ca5(PO4)3F)

sedimentary (fossilized organic matter)

Brachiopod Fossil (Spiriferid)

Order Spiriferida (Phylum Brachiopoda); mineralized as Calcite (CaCO3) or Chert (SiO2)

sedimentary (fossilized organic remain)

Brachiopod Fossil (Spiriferid)

Phylum Brachiopoda, Order Spiriferida (CaCO3 - Calcium Carbonate)

sedimentary (fossilized organic remains)

Kambaba Jasper

Stromatolite (Fossilized Algae)

sedimentary