Yominum

Yominum is apparently an element, symbol Ym. It is apparently radioactive, and presumably a heavy element which, and whose compounds, are thus poisonous.

Spock input the correct molecular formula for yominum sulfide crystals in his training computer after undergoing the reblending of his katra with his body. 

As "yomin" is "nimoy," the Russian word for "mute," spelled backwards, the element may not even actually exist.

ヨミニウム