The Petoskey stone, a distinctive geological specimen, captivates with its unique appearance and ancient history. Popular as a collector’s item and often crafted into decorative pieces, its allure stems from its natural beauty and the story it tells.
The Fossilized Composition
The Petoskey stone is composed of fossilized coral, Hexagonaria percarinata. This colonial rugose coral thrived in warm, shallow seas during the Devonian period, about 350 million years ago. Each hexagon on the stone represents the skeletal remains of an individual coral polyp, called a corallite. These polyps lived in tightly packed communities, forming vast underwater reefs.
Its hexagonal shape and radiating lines within each cell are unique features. At the center of each hexagon, a small dark spot marks where the polyp’s mouth once was, often referred to as the “eye”. This preserved structure of individual polyps creates the intricate, recognizable pattern characteristic of the Petoskey stone.
Geological Journey: From Coral to Stone
Ancient coral transformed into Petoskey stone through fossilization and petrification. After the coral colonies died, their skeletal structures were gradually buried under layers of sediment on the seafloor. This burial protected the organic material from rapid decomposition, initiating the preservation process. Over millions of years, the coral’s calcium carbonate was slowly replaced by minerals, predominantly calcite or silica.
This mineral replacement, known as petrification, preserved the intricate details of the coral’s structure within the newly formed rock. The Devonian seas that once covered Michigan eventually receded, and geological forces, including tectonic activity and uplift, exposed these fossil-rich rock beds. Later, during the Pleistocene epoch, glaciers scoured the land, plucking fragments of fossilized coral from the bedrock and scattering them across Michigan’s lower peninsula. Glacial movement and wave action in lakes further rounded and polished these stones, making their patterns more visible.
Identifying a Petoskey Stone
Identifying a Petoskey stone relies on its distinctive visual characteristics. Its most prominent feature is the hexagonal, or six-sided, pattern. These hexagons are the preserved corallites, each typically displaying a darker center point or “eye” with thin, radiating lines extending outwards.
The stone’s typical colors range from gray and brown, though rare specimens can exhibit pink hues due to iron ore. When dry, a Petoskey stone can resemble ordinary limestone, often appearing dull and unassuming. Wetting or polishing the stone enhances the visibility and contrast of its hexagonal patterns, revealing its intricate beauty.
Michigan’s State Stone and Its Namesake
The Petoskey stone holds special significance in Michigan, having been officially designated as the state stone in 1965. It is predominantly found across Michigan’s northern lower peninsula, particularly along the shores of Lake Michigan and Little Traverse Bay. The stone and the city of Petoskey are named from local history and legend.
Both the stone and city are named after Chief Pet-O-Sega (Ignatius Petosegay), a respected leader of the Ottawa (Odawa) Nation. Born in 1787, his name translates to “rising sun,” “rays of dawn,” or “sunbeams of promise” in the Odawa language. This name is fitting, as the radiating patterns within the stone’s hexagons can resemble sunbeams.