For the moment of the same name: The Philosopher's Stone.
The Philosopher's Stone is a legendary substance made with alchemy. It supposedly turns lead to gold and is used to create the Elixir of Life, which gives the drinker eternal life. In Harry Potter and the Philosopher's Stone, Voldemort attempts to acquire the Stone through Professor Quirrell.
From the Story
"The Philosopher's Stone can turn metal to gold and produce the Elixir of Life, which makes the drinker immortal. The only known Stone in existence during Harry's first year belongs to Nicolas Flamel."
"Voldemort spends most of Harry's first year trying to get the Stone: he forces Professor Quirrell to try to steal it from Gringotts, and, on failing, attempts to steal it from Hogwarts. Harry, however, obtains the Stone first and refuses to hand it over, even when Professor Quirrell attacks him. After Voldemort flees the school, Professor Dumbledore arranges for the Stone to be destroyed."
J. K. Rowling's Thoughts
|“|| I did not invent the concept of the Philosopher's Stone, which is a legendary substance that was once believed to be real, and the true goal of alchemy.