periclase
/ˈpɛrɪkleɪz
/ /ˈpɛrɪkleɪs/noun [mass noun]
A colourless mineral consisting of magnesium oxide, occurring chiefly in marble and limestone.
Origin
Mid 19th century: from modern Latin periclasia, erroneously from Greek peri 'utterly' + klasis 'breaking' (because it cleaves perfectly).