Philosopher’s Stone – aka “Jewel of Time”. An artifact of immense power created by the crystallization of temporal energy. The Doctor described the crystal as a kind of cyst formed by a particular kind of damage to the Web of Time. The Kronos Crystal was a Jewel of Time created by the Guardians of Time and used to trap a Chronovore [OS: The Time Monster]. After that crystal was discovered by the people of Atlantis, legends of its power prompted the alchemists of Earth to attempt to recreate the crystal via a process called the Magnum Opus, a primitive form of Block Transfer Computation [PDA: The Quantum Archangel]. In the 15th century, the alchemist Nicolas Flamel abused Time Lord knowledge he gained from The Doctor to complete the Magnum Opus and produce a Philosopher’s Stone [US: As Above, So Below]. The stone acts as an anchor point for its own independent web of time. A person bound to the stone effectively gains their own private spacetime, rendering them functionally immortal [US: Doctor Who and the Prisoner of Torquemada]. Traveling in time via TARDIS disrupts this private spacetime, causing the user to age rapidly. This private spacetime can also be used to circumvent temporal barriers as were used by the War Lords during the Great Time War [US: War of the Doctor]. Flamel attempted to use the stone to destroy Earth, but was persuaded to redirect its energy in the form of a Rainbow Road into the Time War [US: Doctor Who and the Philadelphia Experiment]. The eighth, ninth and tenth Doctors later used a Schrodinger Cell to create two additional stones, which they used to remove the planet Arcadia from the event horizon of the war. It initially appeared that the stones were consumed in the process, but the eighth Doctor later recovered them and attempted to use them to hide Gallifrey in its own private spacetime [WDA: The Gallifrey Chronicles]. He did not know whether this was successful.
Blonde over blue; your hands are cold, your eyes are fire. -- Billy Joel, Blonde Over Blue