They are used in the "Minatour (sp?) Quest" that you can get in Heartwood. You need to obtain 3 of them, 1 fragment of a map, and 20 gaman horns in order to complete the quest and get a strongbox. It's not a bad quest really, and I do it when I'm bored or between RoT gains.
The only thing is that the Minatour aritfacts are a bit of a pain to get if you don't have stealth (nearly impossible really). You have to stealth deep into the Labyrinth where they spawn on the ground, about 15 or so at a time, kinda like the mushrooms in heartwood.
There are two types. The little pots that are 5 stone and the massive kettle looking ones that weigh 30, as Skylark has lamented. When they are sold they go for around 1k or so per I believe, at least that's what I bought them for when I didn't get them from stealther friends.
