A mineral needed to repair the warp nacelles of USS Voyager, the search for which brings an away team to a moon bearing a symbol from Chakotay's childhood.
Polyferranide appears in the Voyager episode Tattoo. The ship needs a supply to seal its warp coils, and Chakotay leads a survey party on a moon where the deposits turn out to be unsuitable.
There he finds a mark cut into the ground that he last saw as a boy, on an expedition with his father Kolopak in Central America. A warp trail leads Voyager to a nearby planet with far better deposits, but violent storms keep driving the crew away.
Chakotay stays behind and meets the Sky Spirits, the aliens who visited his ancestors tens of thousands of years ago. Once they are satisfied that the visitors mean no harm, they calm the weather and let the ship take the polyferranide it needs.