A Shimmer In The Dark

Shimmer is the pearly pool in the Aether cavern, hidden in the outer underground of every 1.4.4+ world. Toss almost any item in and it transmutes into something else - ores swap tiers, potions become their reverse, and plenty of unique conversions besides. One transmutation pops it. The Aether itself is worth the hunt: shimmered gear underpins several other achievements (New Digs, Training Day).