Answer

All the sets of clamshell stones include black stones made of the highest quality "Nachiguro" slate, mined in Mie Prefecture, near Nagoya Japan. Nachiguro is often used as a touchstone to check the purity of gold. Black stones are matte finished. If its surface is too shiny, players might confuse black and white stones due to optical illusion caused by trick of the light.
The size of black stone and white stone is different so that both stones seem to be totally equal for players. Black stone is 0.3mm bigger and 0.3mm thicker than white stone.