Two notes here.. the color of the sphalerite is more or less a function of the amount of iron present. If there is much iron, the sphalerite will be dark and almost opaque. Second comment - in Tallapoosa County, Alabama, there are exposures of ultramafic rocks that apparently are the result of the alteration of seafloor basalts. These were formed as the proto-Africa approached what is now North America. Much later, in the Triassic, the continental plates parted ways, and small rifts developed all along the east coast. These rifts filled with basaltic flows and in some areas also filled with the famous brown sandstone used in NYC buildings. The southernmost of these rifts are in Alabama but are poorly exposed and small in extent - actually they are dikes.
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