Philosopher's Stones are Forgotten Alchemy Assets that can be acquired as necessary components for completion of a number of rare Forgotten Alchemy tasks (levels 14-20) and/or as optional components in rare tasks that will improve the chances of producing a greater number of potions or dyes from those tasks.
A [Worn Philosopher's Stone] may be purchased for 1 from M'tari Prin'tal in Protector's Enclave (North corner of the Seven Suns Coster Market). Philosopher's Stones of higher tiers may be acquired from [Professions Booster Pack] and asset packs purchased in the Zen store. Philosopher's Stones are not bound and may be traded on the Auction House or transferred between players.
A task requiring a Philosopher's Stone as an asset will take any tier of Philosopher's Stone in its slot. Higher tiered Philosopher's Stones serve only to improve chances of producing additional potions or dyes.
At Profession Level 3, 4x [Iron Philosopher's Stone] can be upgraded to produce a [Steel Philosopher's Stone].
At Profession Level 4, 4x [Steel Philosopher's Stone] can be upgraded to produce a [Mithral Philosopher's Stone].
There is no task that will allow [Worn Philosopher's Stone] to become an [Iron Philosopher's Stone].