Black calcite from Romania is well know; those calcite obtain there black color mostly by inclusions of sulfides (ores).
The calcite crystals on this piece from Santa Eulolia, Mexico, showing multiple grow stages; a clear white core covered with a amorphous mixture of ore-like material and calcite.
Top of course is the cover of fine crystalline bright white quartz over the black calcite.
Sideview shows that the quartzlayer has only a thickness of 3-4 mm. Well, special production of mother-earth shall we say.
Dimensions: 100x70x45 mm; weight 0,300 kilogramme.
Purchased a long time ago in THE minerals store in New York :-)