Kryptonite is a mineral formed from the remains of Superman's former planet, Krypton. It causes Superman to be weakened should it be in the direct vicinity of him.
Bloontonium is a radioactive substance making a debut in BTD5. It is in the form of an upgrade for Dartling Guns, the Monkey Ace Ground Zero ability, and possibly The Big One. It also has its own track named the Bloontonium Lab.
Radioactive waste is commonly seen around Springfield as an ominous green sludge-like liquid, often sealed in leaking drums marked with the radioactive symbol. These barrels are commonly seen due to the lax disposal methods of the Springfield Nuclear Power Plant and Mr. Burns.
It is Splendid's one weakness, and being exposed to it causes him to lose his superpowers and fall ill. When it is taken away, he regains his superpowers fairly quickly. It caused his death in Gems the Breaks after he crushed it into a powder and accidentally inhaled it.
Unobtainium: An imaginary unavailable material used humourously to solve otherwise impossible problems; an item of unaffordable price. Also an upgrade in Cookie Clicker with the name Unobtanium Mouse.
Omnisolve: a thick, black liquid capable of instantly dissolving anything except diamonds, with it then transforming the dissolved matter into a super-dense dust.
Adamantium is a very dense, artificial, iron-based alloy that is virtually indestructible. A sufficient amount is capable of surviving multiple nuclear explosions with no damage.
Jerktonium is a fictional element seen in It's a SpongeBob Christmas. The antidote for it is the tune of the song "Don't Be a Jerk (It's Christmas)". Jerktonium is known to make people act mean and change them into jerks (except SpongeBob, due to his pure heart and small brain). They also have big black bushy eyebrows, black baggy eyes and five o'clock shadow.
First revealed by the story depicted within the Team Fortress 2 comic Loose Canon, Australium is a metal element which can adapt and transform itself into different states and forms, therefore making it extremely valuable and sought-after.