sedimentary

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

Hardness: 3.0-3.5 (Mohs scale); Color: Gray to tan when dry, distinct hexagonal patterns visible when wet; Luster: Dull to earthy (natural), vitreous (polished); Structure: Colonial coral hexagonal cells; Composition: Calcite (CaCO3).

Hardness: 3
Luster: Dull to earthy (natural), vitreous (polished)

Identified March 28, 2026More sedimentary →

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 (approximately 350 million years ago) in warm, shallow saltwater seas. Over millions of years, the coral colonies were buried and fossilized through the replacement of organic matter with calcite, then later moved by glacial action.

Uses & applications

Predominantly used for jewelry making (pendants, beads), lapidary art, home decor, and as a popular souvenir or collector's item in the Great Lakes region.

Geological facts

It is the official state stone of Michigan. The name 'Petoskey' comes from the Ottawa Chief Pet-O-Sega, meaning 'rising sun' or 'rays of hope.' The hexagonal patterns are the former skeletons of individual coral polyps.

Field identification & locations

Identify in the field by looking for 'stones with eyes' along shorelines. They are easiest to spot when wet, as the hexagonal pattern disappears when the stone is dry. Found predominantly on the beaches of Lake Michigan and Lake Huron.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone (Quartz-rich)

Arenite (SiO2)

sedimentary

Sandstone (with man-made markings)

Arenite (composed primarily of Quartz, SiO2)

sedimentary