Tin

Tin is the second-easiest metal to mine. Tin veins also produce gems and, more frequently, Coarse Stone. Tin ores are smelted into tin bars at the forge. In turn, one tin bar can be smelted with one copper bar to make two bronze bars.

Tin Vein
Tin veins can be mined at mining skill level 50. According to Thottbot, tin veins are mined most often in The Barrens.

Tin Bar
Tin bars are ingredients only for the recipe to smelt bronze with copper bars. They are not directly used in blacksmithing or engineering; rather, the bronze bars made are.