Unglazed clay is quite porous - you can use unglazed clay pots as scent diffusers by putting essential oils inside. I would not recommend them for long-term storage.
An awful lot of glazes, on the other hand, contain various toxic chemicals. If you can guarantee that the glaze used was food-grade or food-safe, it should work if you get a good seal - I've seen commercial tobacco jars made of ceramic. I'd still go with glass, myself...