Australium

Australium is a fictional rare, special element which is a reference to the TF2 material of the same name that is visually very similar to Gold. It is special in that it is a trans-schrabidic ultra-heavy element that not only naturally occurs, but is also fully stable and still undergoes fission.

Production
It has a special ore deposit at X: -400, Z: -400 and an average quantity of ~490. Smelting the ore is extremely inefficient as it only returns single nuggets. Shredding gives you 2 powders as per usual, however.

Australium can also be produced in the Cyclotron by bombarding Weidanium with Beryllium.

Uses
Australium is used for the Market Gardner shovel weapon, which deals critical hits to mobs when rocket jumping (or just hitting them when falling from a great height). Has a chance to deliver a nuclear explosion, killing both you, your target, and everything around you.

Euthanasia special syringe gun.

It can be SILEXed into Greater and Lesser Australium isotopes, which gives it use as RBMK fuel.

Mark III Life Extender.

Trivia

 * As an ultra-heavy element that's also trans-schrabidic (elements coming after Schrabidium, atomic number 126), it is miraculously fully stable.
 * Its fission products are also apparently fully stable as well, except for Xenon-135.
 * Its two isotopes have very different melting points, which is very unusual.
 * It is the only special element to retain its place and role after all the others got soft-removed.
 * According to the TF2 wiki, Australium has the same electron configuration as Gold.
 * In the TF2 comics, Australium appeared to be somewhat luminescent, whether this is due to it emitting photons as radioactive decay of some kind or it simply being artistic in choice to emphasize its presence is unknown.