A stone of this nature is typically an oddly-shaped bit of extrusive rock, shaped and roughly polished. 

The possessor of such a stone needs but utter a single command word, and an earth elemental of 12 hit dice size will come to the summoner if earth is available, an 8 hit dice elemental if rough, unhewn stone is the summoning medium. (An earth elemental cannot be summoned from worked stone, but one can be from mud, clay, or even sand, although one from sand is an 8 dice monster also. The area of summoning for an earth elemental must be at least 4-ft square and have 4 cubic yards volume.) 

The elemental will appear in 1-4 rounds. 

For details of elementals and their control see [...] Elemental.

One elemental per day can be summoned by means of the stone.