A silicon-based species encountered in The Devil in the Dark.
The Horta are a silicon-based species discovered on the planet Janus VI. They look like a mass of rock and move through solid stone by secreting a powerful acid, which allows them to tunnel through the ground, and their bodies are covered with a tough, hardened shell.
The Horta are long-lived and have a strange life cycle. The adults die, leaving behind a large number of eggs which are tended by a single parent, the last of its kind, until they hatch. Because they are so different from carbon-based life, humans at first failed to recognise them as living beings at all.
In the original series episode The Devil in the Dark, a lone Horta kills miners at a mining colony on Janus VI, and Kirk and Spock investigate. A mind meld by Spock reveals that the creature is protecting her eggs, which the miners had destroyed. Kirk and the Horta reach an agreement that allows the miners and the Horta to share the planet and work together, and Dr. McCoy repairs the Horta's injury with the help of thermal concrete.