Nukorite

Before the invasion, Nukorite was a faintly magical but unimpressive crystalline material, previously known as willow-glass. It appeared as quartz-like cluster, in transparent greenish and whiteish hues with lattice-like weaves beneath the surface. Its most remarkable trait was that it could be melted and reshaped like metal ore. Being neither uncommon or particularly pretty, nor always easy to get, it was mostly ignored.
But when the aliens invaded, Nukorite showed a remarkable ability to absorb radiation. Charged Nukorite radiated barely contained nuclear fury, with cracks and vents appearing as the charge increase. Initial experiments with Nukorite was simple enough - people dipped the crystal into something that seemed radioactive and threw it at their enemies, discovering that an over-charged piece of Nukorite would explode. From there, Nukorite became much more popular. Since, it's been used as ammunition for slings and catapults, or components for arcane spells. It is a weapon not wielded by the faint of heart, as the radiation that leak from the quartz will poison the hand that holds it just as much as whoever it strikes.
Mechanics
When Nukorite is used to make a weapon or piece of ammunition, they deal an additional 1d6 of Radiation damage. Additionally, Nukorite weapons can be charged; if the user takes Radiation damage while holding the weapon, the weapon becomes unstable. On the next attack, the Nukorite weapon will deal 2d10 extra damage instead of 1d6 in a 5ft radius, then be destroyed.

Allies, or the user themselves, can use abilities with Radiation effects to charge the Nukorite weapon without wounding the wielder.

Every day that a Nukorite weapon is carried, its owner must roll a Constitution check against DC 15, or gain one level of Contamination.
For obvious reasons, Nukorite is never used to make armor, but did have a brief craze as a healing potion additive. (It did not work)

Working Nukorite

As the xenos invasion continues to grind the world to dust beneath its relentless advance, every means to resist them is explored and tested. As the smiths and wizards of the world unlock the secrets of the invader, they have learned to exploit the radiation within the Nukorite to enchant weapons of terrible potency.
Forging Nukorite is even more hazardous than wielding it. Smiths work in lead-lined suits, inside scorching hot chambers choked by poisonous fumes. Once charged, Nukorite loses the malleability that was its most interesting feature before the xenos invasion, so the items are forged into shape first, then charged and quenched. The Nukorite must be charged slowly by exposure to radiation over a period of weeks and repeatedly tempered. Too fast, and the Nukorite becomes unstable. Too slow, and the radiation leaks out faster than it can harden inside the nukorite's crystalline structure.
There are perhaps a dozen crafters who work with Nukorite, and maybe twice that who do it poorly (and often fatally). With the coming of the Way of the Megaton Fist, demand may be increasing.

Finding Nukorite

Before the invasion, Nukorite was found in deserts, arid mountainous areas, and the like. They'd bloom in small clusters at ground level, and rarely inside heated caverns (such as magma springs or sulfur chasms). Prospectors who cared to could break it loose from the ground with a hammer and a chistle, but those days are gone. Nukorite is now found in the same areas, but only after a nuclear attack or similar incident. Deserts that were already dangerous are now irradiated wastelands.
First of all, prospectors must learn to determine if Nukorite is ripe; if it has enough charge to be worth the effort but not so much to render it unstable. It it is suitable at even the lowest levels of radiation, it still requires careful handling to avoid Contamination or rendering it unstable. Foragers are adviced to wear protective gear, suffocating in the heat where the crystals are usually found, and leaden containers for their prize. Some blooms of Nukorite have roots, where the crystals on the surface is just a part of a larger vein. Small, careful cuts to sever suitable chunks of the crystal must be performed with precision, or risk a chain-reaction.
All this means that Nukorite is expensive and items few. Acquiring one is rarely a case of gold and silver, but of barter, favor, and politics.