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Corundum (from Tamil "kurundam") is a crystalline form of aluminum oxide (Al2O2) and one of the rock-forming minerals. It is naturally clear, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red, while all other colors are called sapphire. A pinkish-orange sapphire is called padparadscha.

Due to corundum's hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every other mineral, leaving behind a streak of white on the other mineral. It is commonly used as an abrasive, on everything from sandpaper to large machines used in machining metals, plastics and wood. Some emery is a mix of corundum and other substances, but this mix is less abrasive, with a lower average Mohs hardness near 8.0.

In addition to its hardness, corundum is unusual for its high density of 4.02 g/cm³, which is very high for a transparent mineral composed of the low atomic mass elements aluminium and oxygen.

Corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It also occurs in low silica igneous syenite and nepheline syenite intrusives. Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyre dikes and as large crystals in pegmatites. Because of its hardness and resistance to weathering, it commonly occurs as a detrital mineral in stream and beach sands.

Every possible corundum morphology—bipyramidal, prismatic, tabular, and rhombohedral—was found in inclusions in the igneous matrix of the Forerunner artifact recovered from the Côte d'Azur Museum of Natural History on Sigma Octanus IV. When scanned from tip to base with neutron imagers, a pattern of squares, triangles, bars, and dots was produced.[1]