|“||Well, the key to completing my ensemble maybe.||„|
| — Lucifer to Amenadiel|
in "Sympathy for the Goddess"
Lucifer's ring is a ring that Lucifer Morningstar brought with him to Earth and always carries with him.
This ring, along with the Pentecostal Coin, were the only two things Lucifer brought with him to earth. Lucifer has never taken this ring of his middle finger, and he even wears it when he strips down naked.
In the season 2 finale Lucifer is knocked unconscious and stranded in the desert. All his possession are stripped from him, save for this ring and his pants.