periclase
Concise Oxford English Dictionary © 2008 Oxford University Press:
periclase /ˈpɛrɪkleɪz, -s/
▶noun a colourless mineral consisting of magnesium oxide, occurring chiefly in marble and limestone.
– origin C19: from mod. L. periclasia, erroneously from Gk peri ‘utterly’ + klasis ‘breaking’ (because it cleaves perfectly).
'periclase' also found in these Oxford entries:

