On KingsAge what is the Stone of the Wise

The Stone of the Wise, more commonly known as the Philosopher's Stone, is a legendary chemical that could turn other materials like lead into gold. One of the goals of alchemy was to discover this chemical.

On KingsAge, it is not an actual item in-game. It is referenced as a poetic way of saying that as you level up your Alchemist shop, there are other features that can be unlocked before you can create a Goldsmith and create gold armour (ducats in earlier versions of the game).