Psilomelane is the name of a group of hard black hydrous manganese oxides that has no definite chemical composition but occurs in botryoidal masses and stalactites, and a hardness of 5 to 6 on the mohs scale. 

Gem psilomelane is rare and a popular location for gem material was the Crown of Silver mine in Mexico, however, it was not a large deposit. If you find some you love, grab it up! There's not much of this amazing gemstone.