The Star of Yamatai was a large crystal of unknown composition 25.6 mm across, similar in appearance to a diamond. The crystal appeared to glow from within, which gave rise to it's name. In Japanese legend, the Star was a device used by the Sun Queen Himiko to focus her powers. (Star Trek Online: Valkyries)

In reality, the Star was a fragment of a crystal originally designed to stabilize polaric ion energy during experiments. Bonding to it allowed the polaric-energy based non-corporeal lifeform that assumed Himiko's identity to stabilize itself and amplify its power. The destruction of the Star by Lieutenant Carlin Drel in 2407 effectively killed the entity, causing its energy to build up uncontrollably and catastrophically explode. ("Survivors of Yamatai")

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