MgO is Magnesium Oxide.

The following may tell you more than you need to know, but Magnesium Oxide is actualy very useful / valuable in a number of ways:

Magnesium oxide, or magnesia, is a white solid mineral that occurs naturally as periclase and is a source of magnesium. It has an empirical formula of MgO. It is formed by an ionic bond between one magnesium and one oxygen atom. Magnesium oxide is hygroscopic in nature and care must be taken to protect it from moisture. Magnesium hydroxide forms in the presence of water (MgO + H₂O → Mg(OH)₂), but it can be reversed by heating it to separate moisture.

Applications for MgO

MgO is an efficient moisture absorbent used by many libraries for preserving books. It is also one of the raw materials for making cement in dry process plants; specifically Portland cement. If too much of MgO is added, the cement may become expansive.

Medical

In medicine, magnesium oxide is used for relief for heartburn and sore stomach, as an antacid, magnesium supplement, and as a short-term laxative. It is also used to improve symptoms of indigestion. Side effects of magnesium oxide may include nausea and cramping. In quantities sufficient to obtain a laxative effect, side effects of long-term use include enteroliths resulting in bowel obstruction.

• MgO is used as an insulator in industrial cables, as a basic refractory material for crucibles and as a principal fireproofing ingredient in construction materials. As a construction material, magnesium oxide wallboards have several attractive characteristics: fire resistance, moisture resistance, mold and mildew resistance, and strength.

• It is used as a reference white color in colorimetry, owing to its good diffusing and reflectivity properties. It may be smoked onto the surface of an opaque material to form an integrating sphere.

• It is used extensively in electrical heating as a component of "CalRod"-styled heating elements. There are several mesh sizes available and most commonly used ones are 40 and 80 mesh per the American Foundry Society. The extensive use is due to its high dielectric strength and average thermal conductivity. MgO is usually crushed and compacted with minimal airgaps or voids. The electrical heating industry also experimented with aluminium oxide, but it is not used anymore.

• Pressed MgO is used as an optical material. It is transparent from 0.3 to 7 µm. The refractive index is 1.72 at 1 µm and the Abbe number is 53.58. It is sometimes known by the Eastman Kodak trademarked name Irtran-5, although this designation is long since obsolete. Crystalline pure MgO is available commercially and has small use in infrared optics.

• It is packed around transuranic waste at the Waste Isolation Pilot Plant, to control the solubility of radionuclides.