A projection of the molecular structure of yominum sulfide
Yominum sulfide is apparently a chemical, with a molecular formula of K4Ym3(SO73Es2), that presumably forms crystalline structures.
When Spock was performing a memory test, one of the questions asked him what the molecular formula for yominum sulfide was. (Star Trek IV: The Voyage Home)
According to the Star Trek Encyclopedia, yominium [sic] was "invented" by Kirk Thatcher, who named after Leonard Nimoy – "Yomin" is "Nimoy" spelled backwards.
As the formula is known to be inconsistent with those of known sulfides, the substance may not even exist—and the very memory-test question itself might have been a trick question in that case.