It can be crafted through use of a Pot for fusion or a Pot of alchemist. The recipe can be found rarely in dungeons, or bought from a souvenir vendor, which costs 1,500,000 gold before negotiation price reduction. It requires one potion of mutation, and one of the 6 different evolution heart items. The description says that it takes 30 turns and 10 hours to complete. It requires 50 Alchemy and 150 in your Magic stat.
It is also a reward for completing the Dog Tail quest.
It could be a reference to the Stone Masks in Jojo's Bizarre Adventure.
The sprite for the Philosopher's stone mask is located at 824 (Row 26, Column 32) by the character sprite numbering system.