In RuneScape Classic, this amulet was called Gnome Emerald Amulet of Protection. From its release until the Evolution of Combat, this item gave melee defensive bonuses and was generally used by lower level members. As of now however, it is purely cosmetic.
When the item was first released, players could put it up for trade despite it being untradeable. However, the other player would still not be able to see the item, and finishing the trade would not transfer the item.