Black diamonds are particularly rare gems, similar to yellow, red, blue, green and clear diamonds - they occur only as individual gems within existing clusters of faint yellow diamonds which are themselves found only within kimberlite, itself found only within gabbro layers. Correspondingly, they are among the most valuable gems that can be found in Dwarf Fortress.
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A black diamond will not manufacture cheese in any form. Their presence also does not indicate difficult skiing conditions.