An Amulet of Greater Protection is an item found in the dungeon and can be equipped to the amulet slot. They currently only provide an AC boost. They can be found on floors 6 and below.
As of Recent Updates, amulets can now grant Speed Boosts instead of AC
(Confirm Speed Boosts for Gold Amulets?)
OP = Of Protection
Broken --- OP -2 AC
Worn --- OP -1 AC
Fine --- OP + 1 AC
Excellent --- OP + 2 AC