Star Trek: The Devil in the Dark

On Stardate 3196.1, the starship USS Enterprise, under the command of Captain James T. Kirk, is sent to the pergium mining colony on planet Janus VI to investigate reports of a strange creature that has recently killed 50 miners with a strong corrosive substance and is destroying equipment.
Spock's curiosity is aroused by a spherical object on the desk of the mine supervisor, Chief Engineer Vanderberg, who explains that it is one of thousands of silicon nodules found on recently-opened levels of the mine, but of no commercial value.
Soon, another guard is killed and a circulation pump, vital to the colony's main reactor, is stolen. Scotty constructs a "Heath Robinson" type replacement but the original component must be found within 48 hours or the reactor will fail, rendering the mine uninhabitable. Spock suggests that the creature might be a silicon-based lifeform and would thus be resistant to the "Type I" phasers carried by the colony guards; however, the landing party's "Type II" phasers should be able to stop it.
The landing party starts searching the newly opened Level 23, as it is where all the attacks have taken place. Kirk and Spock soon encounter a creature which looks like an animated clump of molten stone. The creature threatens them and receives a direct phaser blast. Injured, it escapes by burrowing quickly through solid rock. Spock examines a piece of the creature knocked off by the phaser blast and determines that it is silicon-based and secretes a strong acid which allows it to move through rock as easily as we move through air. Spock's tricorder readings show that there is only one creature within a hundred miles and if it is the last of its type, killing it would be a crime against science, but Kirk believes the mine is too important and the creature too dangerous to let it live.
Scotty's efforts fail and the colony begins evacuating, but the landing party and some of the miners remain behind to search for the pump. Kirk discovers a chamber filled with thousands of the silicon nodules and the creature causes the roof to collapse, trapping him. Kirk contacts Spock, who in a reversal of their previous positions, urges Kirk to kill the creature immediately. However, Kirk realises that the creature is not threatening him and begins to "talk" to it. The creature turns, showing him a large wound in its side.
Spock finds a way into the cavern and attempts a Vulcan mind meld but cannot complete it as the creature is in agony from its wound. However, he does learn that it calls itself a Horta. The Horta also gains enough knowledge from the experience to be able to etch the words "NO KILL I" into a nearby rock.
In an effort to win its confidence, Kirk orders Dr. McCoy to come and help the creature while Spock mind melds with it again. He discovers that every 50,000 years the entire race of Horta dies, except one who remains to protect the eggs and act as the mother to them. When the miners broke into the hatchery, it just fought back the only way it knew. McCoy arrives and analyzes the Horta's physiology and finding it is virtually made out of stone, declares: "I'm a doctor, not a bricklayer!" It understands Kirk is trying to help and tells him the missing pump is in the "Vault of Tomorrow".
Meanwhile, Vanderberg and the miners are being held back by a security detail, but their patience wears thin and they overpower the guards. Kirk stops them from killing the creature and explains that the Horta was simply protecting its eggs, the silicon nodules the miners have been collecting and destroying, which are now on the verge of hatching. Vanderberg regrets the damage his men have caused inadvertently, but is concerned that there will be thousands of those "things" crawling around. Kirk returns the pump and reassures him that the Horta are intelligent and peaceful and suggests they could assist the miners by locating new deposits of minerals in exchange for being left alone. McCoy proudly informs them that he's helped the Horta by using thermo-concrete, which is mostly silicon, as a bandage and reckons that he could probably "cure a rainy day!" Spock mind-melds with the Horta again and it agrees with the proposition. Spock mentions that contact with the highly logical Horta is "curiously refreshing".
As the Enterprise prepares to leave orbit, Vanderberg reports that the eggs have started to hatch and they have already hit huge new pergium deposits, as well as gold and platinum. He says that the Horta are not so bad once you get used to their appearance. Spock mentions to Kirk that the Horta also found humanoid appearance revolting, but he got the impression that she found pointed ears the most attractive human characteristic.