Zinc is a metal that can only be smelted from sphalerite. While it can be turned into furniture and 'other objects' at a metalsmith's forge, it is also used to create alloys at a smelter. Zinc is particularly useful if you have magma, as it is an ingredient in the creation of magma-proof nickel silver. Zinc is used in the following recipes at a smelter:
Interestingly, magma is hot enough to boil zinc, and the resulting vapor can be quite dangerous to any creatures nearby.