Belize has some minerals, but nobody has found a deposit large enough for a commercial operation. I think the only commercial mining that has been done has been for limestone. That being said conch pearls, coral, and jade could be considered local semi-precious stones, but there is a chance if you buy one of those that it actually came from some place else (so I would ask).

Also there are definitely artisans in San Pedro that make jewelry with a local flair...