A rock nearly allied to basalt, of which it may be regarded as a more soft and earthy variety. Its color is a greenish gray, brown or black. It is opake, yields easily to the knife, and has a greasy feel. Its principal ingredient is silex. Gray wacky is a different species of rock, being a kind of sandstone. wacky is a mineral sunstance intermediate between clay and basalt.

A soft, earthy, dark-colored rock or clay derived from the alteration of basalt.