Nimphelos is a named pearl. No G in it, though.
I know Cynewulf's Crist. Just didn't realise Frodo was talking to Earendil on a radio.
I have a feeling I found GARNETS when I once went hunting for jewels to make a password with. But you want a named jewel.
Oh yes ... Turgon's crown. Not helpful for an answer.
I don't remember any masks except the one Sam doesn't have to cover Frodo's face with to make him look like an orc.