Stone of controlling earth elementals

A stone of controlling earth elementals was a magical item that could be used to summon and control an earth elemental.

Description
A stone looked like a chunk of roughly polished rock.

Powers
When the stone's command word was spoken, it could summon a subservient earth elemental to the user once per day, provided enough earthlike material was present. It required a minimum of four cubic yards of earth, natural stone, sand, mud, or clay, and the conjured elemental was made of the same material.

Earth, mud, and clay tended to create the strongest elementals, while stone and sand created somewhat weaker ones.