Gem's planet was an inhabited planet in the Minarian star system. This planet, as described by the Vians and named after "Gem", was the homeworld for a species of pre-spaceflight empathetic mutes.
In 2268, after Gem proved she was capable of love and compassion, the Vians saved her and her species from extinction just before the system's primary supernovaed. (TOS: "The Empath")
This planet was only mentioned in dialogue.
The descriptive name is from the episode.