The ancient game of GO, which originated in China, consists of 180 white stones, 180 black stones, and a board with a 19 x 19 grid of lines.

My set weighs 9 kg (9,000 grams) if you don't include the box. The board (a goban in Japanese) weighs 720 grams.
How much does a stone weigh, assuming black and white stones have equal weights?