Perfectly formed floater (terminated all around), this unique formation is gray, or black (if you account for the sandiness), and from Peru, instead of the usual sourcing in Morocco, of the more common red variety. Another name for Selenite is Gypsum, and these are apparently about as big as they get (though museum pieces sometimes get bigger). Spectacular on a stand. The triangle is equilateral, so the Width measurement is the same on all 3 sides.
Each angle of this magnificent piece gives a completely different perspective. The first image shows the region of gemmy translucency; the 2nd show the sheeny side; the 3rd shows the orangy "sunspot" moonscape; and the last, a very icy planet. Quite large.
Extremely unique (and intact, because it's brittle!) cluster of fine needle point orange selenite, which appears to have grown over an earlier cluster.