HIO Ideally you should have a few hundred pounds. Crush it, mix thoroughly, then spread it out, take 1/4, then mix that 1/4 part again Spread it out , take 1/4 again, then take your assay sample from this, saving the rest for comparative assays if needed.
As mentioned by Dave, one rock will only tell you what that rock is composed of, not your vein structure.
Don Jose de La Mancha