Tekasite is a substance.
In 2373, a Changeling posing as Julian Bashir combined tekasite with protomatter and trilithium to make an explosive. The Founder planned to use the explosive to create a supernova in Bajor-B'hava'el to destroy Bajor, Deep Space 9 and a Starfleet, Klingon and Romulan force massing at the station. (DS9 episode: "By Inferno's Light")
In 2409, the SS Usa-ken followed an offer to deliver a load of tekasite to Gasko Station. At the starbase, the Tal Shiar asked to come aboard to check cargo and crew manifest. However, the Tal Shiar operatives started shooting all crewmembers except for pilot Rinna Khev. Her brother Tovan was accused of colluding with the Romulan Republic and considered a terrorist. Rinna was taken to Installation 18 on Nimbus III. (STO - "Wasteland" mission: "Installation 18")