Tin

Tin is a metallic chemical element, having the symbol Sn and the atomic number of 50. Tin played a significant role in human history when different cultures discovered that it could be alloyed with copper to create bronze, at that time the hardest metal which humans were able to manipulate. This began the Bronze Age.

Tin in "The Horse of Bronze"
Tin was mined by the Nuggies on the Tin Isle and was their principle export and the centaurs were their primary trading partner for the metal.

One day tin shipments promptly stopped arriving, prompting the centaur Cheiron to lead an expedition to the Tin Isle to attempt to reestablish contact with the Nuggies. He found that the Nuggies had gone virtually extinct and their tin mines lay abandoned. He persuaded the new dominant species of the Tin Isle, the mans, to trade with the centaurs, but the trade agreement was abrogated when centaurs, under the influence of cerevisia, brawled with the mans at a banquet to celebrate their new partnership and had to flee the Tin Isle for their lives.

The centaurs never did find a new source of tin; an expedition into the unexplored interior of Eastern Europe led by Pholus failed to uncover any.