Clay. It's fairly useless right now, but it shouldn't be. The same goes for the kiln - as far as I know it's only used for making pearlash right now. What if you could use clay to create ceramic goods in in kiln? For example bricks(commonly used in housing throughout history), pipes, statues, crafts, even bowls and plates for eating. They would be automatically assigned to a table and would boost the happy thought received from eating slightly. Hell, you could even add cutlery - silver spoons anyone?
Clay would be collected kind of like how sand is right now, probably in buckets. The advantage of clay would be that you don't need to make a "raw" intermediary product - the dwarf simply needs to form the item and fire it up in the kiln.
The basic product would be stoneware. Simple and functional, it would only require clay. Its base value would be low and its main use would be for building items like bricks and pipes.
Porcelain is to stoneware as clear glass is to green glass. It would require materials such as kaolinite, quartz, bone or other ingredients in addition to clay(see http://en.wikipedia.org/wiki/Porcelain#Materials). It would obviously have a higher value and could possibly also be painted with dyes to add additional value.
The production process could either use existing skills(glassmaking, stonecrafting?) or perhaps a new one(potter, clayworker?). Up to Toady, of course.
[ May 28, 2008: Message edited by: Doniazade ]