What do we call a polished stone that is worshipped as a symbol of Shiva?

Linga

Conical stone objects have been identified as lingas, associated with Shiva worship.

This is what Ernes Mackay said about Linga- "Various small cones made of lapis lazuli, jasper, chalcedony, and other stones, most beautifully cut and finished, and less than two inches in height, are also thought to be lingas … on the other hand, it is just as possible that they were used in the board-games … "