Gem of Amarra
The Gem of Amarra was described as the equivalent to the Holy Grail for vampires. This artifact would grant the vampire wearer added strength and complete invulnerability to all their usual weaknesses. For instance, if the vampire was staked (which would traditionally 'kill' the vampire), the wound would close and they would not 'dust' (perish). Also, when the gem was worn, the vampire had the ability to travel around outside during daytime, without the sun's rays harming them. Therefore, the gem was considered by many vampires to be a very desirable item.
The Gem of Amarra was sought out by Spike in season 4 of Buffy the Vampire Slayer. Harmony, Spike's girlfriend at the time, finds a ring with the gem set into it and puts it on. Spike subsequently gets angry and stakes her, but she is unharmed. This leads to Spike taking the ring and facing off with Buffy. During their fight, Buffy takes the ring from Spike, Spike only just managing to retreat to a sewer before the sun destroyed him. Buffy finally sends the ring to Angel via Oz. ("The Harsh Light of Day") Spike follows Oz to LA where he attempts to regain custody of the ring. Despite the intervention of the child-torturing vampire Marcus, who takes the ring and nearly uses it to attack a scout troop, Angel triumphs and uses the ring temporarily, allowing him to witness his first sunset in over two centuries. However, in the end, Angel decides that he cannot keep the ring, both because it is too tempting a prize for other vampires and because it is his destiny to fight for those who live in the darkness and cannot turn to the world of the daylight for help; by spending too long in the 'day' world, he might forget those who truly need him. Angel destroys the gem by smashing the ring with a brick. ("In the Dark")