I recently borrowed a bobcat an relocated some dirt around my house a few months ago an decided to detect it this is what I came up with not sure what the square glass block with metal frame is
either something to a pair of big binoclures, or a big teliscope, i took my broken pair of binoculers apart and got a similer peice of glass like that.
I believe it to be a prism from some type of optics as was suggested by detectingarizona. My older brother use to send home all sizes and shapes of these from Vietnam, They came from various discarded telescopes and the such.