#1, #2, #3) I posted about a couple years back. It's actually a frustum, in this case a pyramidal frustum, one based on an oblique, irregular, triangular pyramid. That is, it's a "chopped" triangular pyramid.
The rock itself was on an alluvial fan not far from Walker Lake; it was pointed out to me by MOH, who spotted it on our second-day hike to some Pleistocene shorelines. Although it was lying on a grusy soil amidst large granitic boulders, the rock looks porphyritic. I don't know if it's a porphyritic version of the granitic intrusive rock lying around, or if it's extrusive in origin.