A lapis lazuli is a mystical stone enchanted by witches, to allow vampires to walk freely in the sunlight. Throughout the series, female vampires often wear amulets, rings or bracelets that possess lapis lazuli stones, as oppose to male vampires, who often only wear rings. The stone itself doesn't have to be worn; for as long as it the Vampire is in contact with it, they are protected from daylight. The charm placed by Witches over the stone is not dependent on the caster, since the Lapis Lazuli have been known to protect their Vampire wearers since long after their witch casters have died. It was implied by Katherine that the stone only protects the Vampire to whom it was enchanted for by the witch that casted the spell. Though somewhat common in the series, very few humans know of its existence and function, which gives vampires the advantage of walking in the sun, avoiding suspicion from most humans who still believe vampires can only walk at night. When Stefan and Damon were interrogated about how they walk in the sunlight on separate occasions by Sheriff Forbes and Logan Fell, they both refused to tell their secret.
In the books, it seems that the power of Lapis Lazuli is transferable, as Elena wears Katherine's ring without any problems.
In the series Stefan's Diaries, Lexi actually borrows Stefans ring. These books are based off of the TV series.
In the series, it seemed to be non-transferrable, as when Lexi asks to borrow Stefan's ring in 162 Candles, he states "you know it doesn't work that way".
In American Gothic, it was confirmed that daylight rings are not transferable. Elena in an attempt to look more like Katherine, asks her to hand over her bracelet. She claimed that she couldn't because it was daytime bracelet which use to be a necklace and it wouldn't work on her either way.