Kelbonite

From Memory Alpha, the free Star Trek reference.

Kelbonite is a refractory metal capable of deflecting or interfering with certain types of scans and energy beams, including the transporter. It can be found naturally within caverns and tunnels on many planets.

Kelbonite alone was not enough to prevent the Crystalline Entity from locating and absorbing organic matter, as tunnels on Forlat III lined with kelbonite offered no protection. However, it was believed that kelbonite in combination with fistrium blocked the Entity's ability to collect organic matter, as colonists on Melona IV survived an attack by hiding within caves abundant in both metals. (TNG: "Silicon Avatar")

The Ba'ku world had mountains containing kelbonite ore. These mountains were used as refuge in order to prevent the Son'a from forcefully removing the Ba'ku from their home. (Star Trek: Insurrection)