This was hard to photograph. The side shown in first 2 pics is clear as glass and reflects any color close by. It appears to consist of rectangular parallel sections. The back side, 3rd pic looks like dried sack-crete or cement. The rock was reflecting sunlight on a gravel road and really stood out while walking past. The roads are maintained with rock from a nearby gravel quarry. I looked for more pieces, no luck. Not sure if rock or mineral.