Beryllium is a truly amazing metal (or metalloid, as some like to call it). Not only is it extremely light, it is elastic yet stiff. It also has an extremely high heating value, yielding specific impulses of up to 750 seconds when coupled with liquid oxygen in a rocket. It is extracted from the mineral beryl. The two disadvantages of beryllium are that it is extremely poisonous, and very expensive, costing around $150/pound.