A naturally occurring mineral whose deposits scatter sensor scans and prevent transporter locks, twice turning up in rock that sheltered people from a threat overhead.
Kelbonite is first mentioned in the episode Silicon Avatar. When the Crystalline Entity strips the surface of Melona IV, Riker, Data, Doctor Crusher and a group of colonists take refuge in nearby caves and survive. The rock contains kelbonite and fistrium, which evidently hid them from the creature.
The mineral returns in Star Trek: Insurrection. As Picard's officers lead the Ba'ku out of their village, they head for hills rich in kelbonite, knowing that the deposits will stop the Son'a from simply beaming the villagers away.
The Son'a answer by sending drones to tag people individually. In both stories the mineral serves the same purpose, giving those on the ground a place that technology in orbit cannot reach.