The gift shop at the GF, YC, and CR all have a selection of precious and semi-precious stones and pearls. I've also seen nice pieces at the watch shop in MGM.
I've got a pair of sapphire Mickey earrings given to me as a gift by someone who used a substantial CM discount when they were purchased. I'll warn you right now, those earrings were (I think) $425 pre-discount, so you're paying through the nose for an icon.
You can also buy Mikimoto pearls in the depaertment store in Japan in Epcot, but you can find them for much better prices outside of WDW.
Anne