A noun for the green precious stone. It can modify another noun with の, as in エメラルドの指輪.
彼女はエメラルドの指輪をしている。
She is wearing an emerald ring.
このエメラルドは色がとても鮮やかだ。
This emerald has a very vivid color.
A large emerald is set into the ring.
An emerald sparkled at her neckline.