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From Wikipedia: A Petoskey stone is a rock and a fossil, often pebble-shaped, that is composed of a fossilized coral, Hexagonaria percarinata. by Mark Willocks via flickr

Wall "mural" of real Knightia and Diplomystus fossilized fish. On auction in Bonham's "Distinguished Fossils" sale.

Igneous rocks formed from lava flows of the Archaean era (>2,700 million years ago) are often found to contain disequilibrium-textured crystals characterized by spherulitic, branching or dendritic morphologies that occur in layers near the flow surface.

opalized and pyritized ammonite fossil one of my favorite of all mineral forms.. just glorious!