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.
TUC14-20 Quartz var. Amethyst Piedra Prieta, Las Vigas, Mexico Jason New Small Cabinet, 8.1 x 6.8 x 4.7 cm Aesthetically perched on a minor amount of matrix, are four crystals of lustrous and translucent, lavender colored amethyst, to 6.3 cm in length. All four crystals exhibit beautiful sceptering at their bases, making this an extremely aesthetic example from these finds. From the personal collection of Jason New, a longtime importer and collector of Mexican material.