The Rooster Crown (spoilers all RotE books)

[Bink]

Perhaps the gems has nothing to do with minstrels at all. If I remember correctly, the minstrels are represented as being in the feathers and not in the crown at all. The crown merely serves as a conduit for the minstrels to collectively communicate to the wearer. This was the source of the minstrels' anguish as none of them could speak independently save through the Fool who used his blood to mend the crown.

As to why gems no longer 'wink', I would think the damage to the crown caused it.

EDIT: I remember now that each of the minstrels was indicated to be favoured of dragons, so much so that the dragons gifted them with 'womb wood' and they were to mix their blood well into the wood while thinking of a feather.