Every now and then, instead of getting a weapon gem over SpotPass, Palutena sends over a ??? gem instead. They can't be converted into weapons, they can't be fused, and selling them provides a measly 5 hearts.
If I had to guess, these gems represent locked weapons (e.g. Zodiac weapons) and will reveal their "true forms" once said weapons are unlocked. Is this it, or something else entirely?