Petoskey Stone is a beautifully detailed fossil stone with a honeycomb pattern. They are grey, brown or beige in color and found only in the Great Lakes region. It is formed of Calcite and has a hardness of 4 to 5. Petoskey Stone is the offical Michigan State Stone - found on the beaches of Lake Michigan and Lake Huron and anywhere in the area where rocks and stones are exposed.
Petoskey Stone is a fossilization of Hexagonaria percarinata , a colonial tetra-coral which lived about 350 - 415 million years ago in the Devonian and Silurian Periods - about 100 million years before the dinosaurs roamed the earth. They lived in what is now the State of Michigan and surrounding area.
At that time, Michigan was positioned near the equator and was covered by a warm shallow sea in which the coral lived in colonies. Corals begin as floating planulae. Hexagonaria find a solid surface on the sea bottom and begin to grow a hard covering called an exoskeleton. The individual coral, called a "polyp", created a six-sided cup as it grew, and spread by producing buds which produced their own six-sided cups surrounding the first. In this way, the colony would spread up and out, forming a dome. The top of the colony will have more evenly hexagonal shaped pattern, and the bottom will typically show a single centered cup surrounded by elongated radial columns. The cups are filled with radial lines, called septa, partitions which provided support for soft coral body. Hexagonaria had a symbiotic relationship with algae. The algae helped the coral by producing photo-synthesis, which used carbon dioxide provided by the coral, and returned needed oxygen and nutrients. The coral and algae needed warmth and sunlight to live which only penatrates the water to a depth of 200 feet. They sifted plankton from water at the sea bottom with tentacles which were armed with stinging cells.
The detail of coral is preserved by the process of petrifaction. After the coral dies, it may be buried by sediments and mud. Calcite, silicon, and other minerals slowly replace the original living material cell by cell, changing it to stone. The Michigan corals were exposed to crude oil at some point in their past, which gives them their brown color, with concentrations of oil at the center of the cups. About 70,000 years ago, glaciers covered much of Michigan and picked up the Petoskey Stones, scattering them widely about the area of Petoskey, Michigan, which gives them their name.