The Philosopher's Stone was one of the first artefacts Lara found. In Tomb Raider: Chronicles, she travels to Rome to search for it. The stone lay within the Philosopher's Gate that could only be unlocked by retrieving the God Stones, and defeating the Gargoyle Serpents that guarded it.
Lara tried to remove the stone from its resting place in a wall of gold. The platform she was standing on gave way, and she was cast into a long tunnel that led to the catacombs of the Colosseum. Lara then found another way back into the temple through the Colosseum. She found and retrieved the stone with success.
The Philosopher's Stone can transmute any metal into gold. Maybe it could also make the Elixir of Life.