Star Crystal

 was a "clear, bluish crystal" which shone "with a liquid, shifting light". It could be used to travel between universes. Votri, the leader of some Tellac Inc. miners, had chanced across the substance, and he and his men had vowed to keep it from their employers. They worried that Tellac Inc. would use it purely for profit, ruining each new universe as they encountered it. Votri entreated K9, who had been hired to find the miners and their missing planet, not to reveal their secret discovery to either Tellac or the Time Lords. K9 agreed, thereby allowing the miners to use the crystals as they saw fit. (AK9: K9 and the Missing Planet)
 * ''Author Dave Martin chose to treat "Star Crystal" as a proper noun, capitalising both words.