From outside, there is nothing special about a dingy workshop tucked away on Puspiptek Boulevard in Serpong, Tangerang. It is made of bamboo, with a roof made of thatched leaves. The walls are also semi-permanent. "This used to be an indoor football field, said one worker at the workshop when Tempo visited there in mid-July.

A huge grinding machine, nearly three meters high, stands in one corner. Stones with blue marbling are in a pile beneath it.