Iridium

Iridium was an element used in the repair of starships. It had a very brief half-life.

In 2262, the SS Beagle was in desperate need of iridium after suffering damage. Captain Merik decided to enter orbit of 892-IV and sent down a landing party to find some iridium, but the local people, who were similar to the ancient Romans of Earth, imprisoned and killed the away team. 

The Nechisti shrine on the Nechani homeworld contained traces of iridium ions. When the USS Voyager visited the planet in 2373, The Doctor determined that they played a key role in protecting the Nechisti Order monks from the biogenic field of the shrine, by inducing a temporary dielectric effect in their outer epidermis that neutralized some of the energy. This phenomenon also protected Kathryn Janeway as she carried Kes into the shrine to be healed. 

In 2376, Seven of Nine and Tuvok attempted to undertake a mission to study a micro-nebula 1.6 light years from Norcadia Prime. Some of the readings they took from Voyager showed iridium oxide particulates, however Penks ship kidnapped them before they could study it. 

In 2377, Voyager traded with a Dralian named Gar for high-grade iridium before he stole The Doctor's program. Tuvok surmised that since iridium has a brief half-life, and Gar's ship was slow, he must have gotten it within three light years. They then discovered a Telsian iridium mine on an asteroid from which Gar had stolen 20,000 kilograms of iridium ore.