Hehehe! It looks like it could be plastic. Sorry, I couldn't resist.
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Sure looks allot like "manganese". Your description also sounds like manganese.It is non-magnetic is often mistaken for stone or concrete.But its hard to say for sure from the picture.But my money is on manganese
it resembles a sink...I tyhink it is a zinc lined sink basin. Made of wood or stone and wrapped with zinc. Are we taking bets yet?
It's concrete. Look at the chip on the bottom in the last picture.