Nature's Gift

It grows as a rare flower on jungle grass underground, and it looks like a blue version of the red rose you find in the same place. Cut it with whatever is in your hand while you tunnel. If you swing a sickle at it you also walk away with hay, which is worth remembering if you are building a farm anyway.

The mana discount on its own is small enough that a dedicated accessory slot is hard to justify. The reason to want the flower is what it becomes: combined with a mana potion it turns into an accessory that keeps the discount and drinks for you automatically the moment you run dry. That is what actually changes how a mage plays, since you stop watching the blue bar and start watching the fight.

Spare flowers are worth crafting up before you sell them, because the finished accessory fetches noticeably more than its two parts do separately.

In the earliest versions of the game it did something completely different, restoring you to full health and mana when you respawned.