The rock was collected from the forebay tunnel which is over 500 feet underground. It has never seen daylight until that day. It used to be much further underground because that area is the top of a volcano formed on a fault line. The bottom portion is the Wilson Range in Arizona while the top portion slide off creating the River Mountains.
The rock is a Mica Shist (metamorphic) with a quartz vein intruding through a weak portion between layers with a bit of pyrite in the quartz. The yellowish brown portions is a ferrous hydroxide which is the mineral goethite.
Link to original pictures
