Fistrium is a mineral element which interferes with sensor readings.
The cavern walls of Trill's Caves of Mak'ala were composed of fistrium. (DS9 novel: Trill: Unjoined)
Fistrium in the caves of Melona IV, in combination with kelbonite, was assumed to have shield the Federation colonists on that planet from being detected by the Crystalline Entity as it attacked. (TNG episode: "Silicon Avatar")
Fistrium in the caves of the Loneel Mountains on Betazed prevented Dominion forces from discovering members of the Betazed resistance who had taken refuge there. (TNG novel: The Battle of Betazed)