It was the central artifact in the first book, Harry Potter and the Sorcerer's Stone (Philosopher's Stone in the UK). After Voldemort's attempt to steal it, the Stone was destroyed by Dumbledore.
The Deathly Hallows focus on the titular Deathly Hallows, which are three powerful magical objects: the Elder Wand, the Resurrection Stone, and the Cloak of Invisibility.