Philosopher's Stone

There is no recipe. Mimics drop it, so it arrives when it arrives, usually while you are doing something else in the caverns during early Hardmode. If you want to force the issue, hunting Mimics on purpose is the only lever you have.

The saving on any one potion is a few seconds, which sounds like nothing until you count how many heals a long fight actually eats. Spread across a full attempt it works out to a quiet, permanent trickle of extra health per second. The stronger the potion you are drinking, the more that trickle is worth, so the accessory grows on you as you move up healing tiers rather than fading out.

The cooldown is recalculated live rather than fixed at the moment you drink. Put the stone on while sickness is already ticking and what remains shrinks in proportion. Take it off and what remains grows back toward the untrimmed wait. Drinking with it equipped and immediately swapping to something else hands you the full cooldown, so there is no free accessory slot hiding in the mechanic. Worth knowing if you juggle loadouts mid fight.

Anything that puts you into Potion Sickness gets trimmed by the same fraction. Eating a Mushroom underground, leaning on Restoration Potions through early Hardmode, or drinking a Strange Brew all come back sooner. Treat it as a general cooldown reduction rather than a healing item.

The moment you can build a Charm of Myths, the stone stops being an accessory and becomes an ingredient. The Charm already contains this effect and the two do not stack, so wearing both is a slot thrown away.

The real thing was the goal of centuries of alchemy, a substance that would turn cheap metal into gold and, in the same breath, keep the person holding it alive indefinitely. Terraria took the second half and dropped the first. It transmutes nothing, but it does let you patch yourself up more often, and the glassy red-purple look of the sprite matches how the old texts described the finished stone.