Silver is a precious material used mainly as coin to trade, and for construction. In the beginning of a new embark (default scenarios only), a starter amount is provided among the wreckage of colonists pods. Silver can be found through mining, drilling and looting raiders.
Trader visitors carry a limited amount themselves. As well as other items such as resources, slaves and weapons. Traders will be specially interested in buying resources relevant to their specialization, such as pirates for slaves and bulk merchants for raw materials.