sedimentary
Petoskey Stone (Fossilized Coral)
Hexagonaria percarinata
AI-generated identification · may be incorrectHardness: 3-4 (Mohs), Color: Gray to tan (often appears reddish or orange if wet or stained by minerals), Luster: Dull to vitreous, Crystal Structure: Hexagonal patterns (corallites), Specific Gravity: 2.7
Identified More sedimentary →
Explore Petoskey Stone (Fossilized Coral) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 3-4 (Mohs), Color: Gray to tan (often appears reddish or orange if wet or stained by minerals), Luster: Dull to vitreous, Crystal Structure: Hexagonal patterns (corallites), Specific Gravity: 2.7
Formation & geological history
Formed from fossilized rugose corals (Hexagonaria) that lived in warm Devonian seas approximately 350 to 400 million years ago. These colonies were buried in sediment and slowly mineralized into limestone.
Uses & applications
Used primarily in jewelry-making (beads, pendants), decorative stone carving, and as the official state stone of Michigan for local collecting.
Geological facts
The name comes from the Ottawa Chief Petosegay. The stone is unique because the hexagonal pattern is only visible when the stone is wet or polished; otherwise, it looks like an ordinary gray rock.
Field identification & locations
Can be identified by the 'spiderweb' or hexagonal pattern across the surface. Commonly found along the shores of Lake Michigan and Lake Huron, especially after storms turn over the lakebed.
More like this
Other sedimentary specimens
Fossiliferous Sedimentary Rock
Sedimentary limestone with invertebrate fossil imprint
fossil
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Quartz pebble
Silicon dioxide (SiO2)
sedimentary
Generic Pebble / Rock Fragment
Lithic fragment / Weathered rock
sedimentary
Sedimentary Sandstone
Arenite (Sedimentary Rock)
sedimentary
Shale (or Mudstone/Siltstone)
Sedimentary Mudrock (SiO2, Al2O3 base with clay minerals)
sedimentary