Stardate: 3196.1
Original airdate: 3/9/1967
Writer: Gene L. Coon
Director: Joseph Pevney

Guest stars:

Ken Lynch               Chief Administrator Vandenberg
Barry Russo             Giotto, Security Chief (also played Com. Wesley in Ultimate Computer)

Brad Weston             Ed AppelChief Processing Engineer
Biff Elliot             Schmitter, miner, burnt to a crisp along with 50 other miners
Janos Prohaska          Horta, silicon based life form, secretes  a corosive acid, lays eggs of pure silicon, 50,000 year life cycle, intelligent, peaceful, mild, harbors ill will towards no one.

Crew appearances:
Kirk, Spock, McCoy
Scott
No Uhura
No Sulu
No chekov

Story:
When miners on Janus VI are being murdered by a creature that can move through solid rock, chief engineer Vandenberg contacts the Enterprise and asks for assistance in hunting down and putting an end to the threat. The starship arrives with Kirk and Spock leading a landing party to investigate the situation. What they discover is that this so-called creature is actually an intelligent creature known as the Horta and that it has only killed to protect its young, which are encased in silicone shells that the miners mistook for worthless rocks.

Pergium production station on Janus 6
Pergium

PXK pergium reaction pump stolen by Horta to rid the palnet of humans

Quotes:
"A lot can happen in 3 minutes" Schmitter
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"I'm a doctor, not a bricklayer." McCoy
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No Kill I printed on rock by the Horta
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"By golly Jim I'm beginning to think I can cure a rainy day." McCoy
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Captain, I see no reason to stand here and be insulted." Spock