Fic Idea:
SpoilerTwilight develops a variation of Rarity's gem seeking spell to find treasure, and while she and Spike are testing it, they find what appears to be a worthless rock. Twilight is discouraged and goes back home to rework it. Spike accidentally wishes for a tasty gemstone and gets one, leading him to figure that the stone is a magic wishing stone. Shenanigans ensue.
I call it "If Wishes Were Ponies..."