Marcasite (a sulfide mineral sometimes called white iron pyrite) with Pyrite, Arsenopyrite, Calcite and Dolomite. Marcasite and Pyrite are polymorphs. They have the same mineral composition but different crystal structures. Pyrite has a cubic structure and is hard enough to be used in jewelry, while Marcasite has an unstable orthorhombic crystal structure and is liable to break apart.
dogtooth calcite crystals atop the amethystine substrate Hubei province of China