Rare earth element
A rare earth element was a very valuable type of chemical element. One example was europium. ()

In 2267, following Spock's breakthrough with the Mother Horta and the subsequent agreement that the Janus VI colony personnel and the Horta should work together to mine the planet, Chief engineer Vanderberg reported that he was "afraid to tell you how much gold and platinum and rare earths we've uncovered." ()

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Geology