A projection of the molecular structure of yominum sulfide
Yominum sulfide was a substance that occurred in crystallized form with the the molecular formula of K4Ym3(SO73Es2).
When Spock was taking a memory test in 2286, one of the questions asked him what the molecular formula for yominum sulfide crystals was. (Star Trek IV: The Voyage Home)
According to the Star Trek Encyclopedia, 4th ed., vol. 2, p. 509 yominium was "invented" by Kirk Thatcher, who named it after Leonard Nimoy – "Yomin" is "Nimoy" spelled backwards.