Pyroxenite is often associated with its ultramafic cousin peridotite. Rocks like these originate deep in the seafloor, underneath the basalt that makes up the upper oceanic crust. They occur on land where slabs of oceanic crust become attached to continents, that is, at subduction zones.
Identifying this specimen, from the Feather River Ultramafics of the Sierra Nevada, was largely a process of elimination. It attracts a magnet, probably due to fine-grained magnetite, but the visible minerals are translucent with a strong cleavage. I was reliably informed that the locality contained ultramafics. Greenish olivine and black hornblende are absent, and the hardness of 5.5 also ruled out these minerals as well as the feldspars. Without large crystals, a blowpipe and chemicals for simple lab tests, or the ability to make thin sections, this is as far as the amateur can go sometimes.