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Rough vs. Polished Stones
Petoskey stones found "in the wild" usually have been polished round and smooth by the glaciers. They frequently have a matte surface revealing a hexagonal pattern when wet. However, some Petoskey stones can be found in the rough.
Hexagonaria Genus
Seven genus of the colonial coral creatures, Hexagonaria, make up the fossil in the Petoskey stones. Typically, it takes a professional to be able to distinguish the types of Hexagonaria.
Mineral Impurities
Petoskey stones consist of calcite, but other minerals like quartz and pyrite can also be present. In addition, the corals themselves can have clay embedded in them. This makes for very different appearances of the stones, and also makes them different to work with depending on what types of mineral contaminants are present.
Stone Hardness
The coral fossils are naturally soft, and impurities make the stones even softer. However, occasionally a hard stone consisting entirely of calcite can be found. The hardness of the stone provides a grading by which different types of Petoskey stones can be recognized.