Sandstone (20 kg) is a quest item used during the Enakhra's Lament quest. It is obtained by speaking to Lazim in the desert south of the Bandit Camp and then giving exactly 20 kg of individual sandstone blocks. The blocks can be mined from the quarry immediately south west of Lazim.
After giving the blocks to Lazim he will magically join them to create the 20 kg block. The player can then use a chisel on this to make the sandstone body for the statue needed in the quest.