I loaded up an old save and found it (with a little help from something I read ). It's called the "Ring of Greed", and it gives you +15 Thievery and +5 Silver Tongue.

It's on Antigua in a chest on the beach. Go left from the ship and follow the beach until it curves and you reach a dead end.

*Edit* Doh… you wanted an amulet, not another ring. Sorry about that.

There is no amulet that gives +15 to thievery. I double-checked by using a console command to give me every amulet in the game. There's only the Thief's Amulet.