Alexandrithium
A unique fictional metal I once created for worldbuilding reasons.
In many worlds I have complicated extensions of common physics to give me more room for freedom, may it be levitation, energy generation, or else.
Alexandrithium, as far as I'm concerned, didn't appear anywhere else as a concept. It's a copper-like material with the lightness of aluminum and strength of crude steel, it possesses a characteristic shine to it, sometimes described as "oil spots on water".
It's greatest property is that it boils water upon contact.
Meaning, once in contact with water, a chemical exchange of energy happens, leading to physical reaction where energy is generated and instantly redirected into the water.
Alexandrithium is a combination of the words "lithium" and the name "Alexander", probably in the honor of the man who discovered it (free for interpretation).
Alexandrithium, as such, can be considered alternative copper, meaning it uses alternative atomic arrangement (fantasy physics thing). Other than it's water-boiling property, it is also highly conductive for both heat and electricity.
It is frequently used in camping gear, such as kettles that don't need fire to boil water, or just small plates of it used as miniature heating elements. It's chemical compounds also keep most of the water-boiling property, so it can be used in explosives, heaters and chemical accelerators.
But, as far as history is concerned, it was used in weaponry too, including knives and bullets, not talking about explosive missiles and strong grenades akin to those made of phosphorus.