periclase

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A mineral form of magnesium oxide, MgO, usually occurring in cubic crystals or grains.

Century Dictionary and Cyclopedia

1. n. A rare mineral consisting of magnesia (MgO) with a little iron protoxid. It occurs in minute greenish octahedrons embedded in ejected masses of crystalline limestone at Vesuvius, and has also been found recently in Sweden.

Wiktionary

1. n. mineralogy A cubic crystalline form of magnesium oxide (MgO)

GNU Webster's 1913

1. n. (Min.) A grayish or dark green mineral, consisting essentially of magnesia (magnesium oxide), occurring in granular forms or in isometric crystals.

WordNet 3.0

1. n. a white solid mineral that occurs naturally as periclase; a source of magnesium