Heatinks have been made out of a whole host of different metals.
Some metals have better thermal disappation characteristics than others.
Aluminium is the most popular heatsink material in use today. It is not the best heat conductor but it is very efficient and cost effective. Silver is the most efficient conductor of heat but constructing a whole heatsink out of silver would be astronomical in production costs. This is why companies have mainly been using the cheaper alternative, aluminium or the more expensive copper. Copper is also a very soft material and would easily be damaged in shipping.