Asbestos

Asbestos was a group of impure magnesium silicate minerals which occurred in fibrous form.

After Kirk and Spock had an encounter with the Mother Horta on Janus VI in 2267, resulting in their phasering off a piece of the creature, Kirk observed that what they removed was not animal tissue. Upon further observation, Spock's closest approximation of what the material was, was that of fibrous asbestos, supporting his theory that the Horta was a silicon-based lifeform.