Periclase is sometimes used as a gemstone although it lacks good hardness and is generally limited in colors. Periclase is isostructural with halite, NaCl, which has simple cubic packing and produces cubic and octahedral crystals.
Periclase is relatively scarce and is found in marbles being formed from the dissolution of dolomite, CaMg(CO3)2 , into MgO-periclase, CaCO3-calcite and CO2-carbon dioxide. This dissolution takes place during metamorphism. If the periclase rich marbles are exposed to weathering the periclase easily alters to either brucite, Mg(OH)2 or hydromagnesite, Mg5(CO3)4(OH)2 - 4H2O.
The yellow brown and black colors of periclase are due to the presence of iron. Specimens of periclase can be very attractive as brightly lustered, smoothly faceted crystals projecting out of the otherwise formless rough marble host rock.
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