Corundum is aluminum oxide - Al2O3. At H≡9, it is the hardest common mineral, apart from diamond. Corundum forms hexagonal crystals, which is evident even in many river-worn specimens.
The hexagonal columns of corundum typically have well-developed flat tops & bottoms. These flat ends are not cleavage planes. Corundum has no cleavage. The cleavage-looking flat tops & bottoms of corundum are called partings (pseudocleavage). Additional breakages will not be along planar surfaces.
The color of corundum is variable - it can be any color, including plaid patterns. If transparent and relatively fracture- & inclusion-free, corundum is said to be of gem-quality, and the color determines the name of the gem.
deep red = ruby
blue = sapphire
pale red = sapphire
pale green = sapphire
purple = sapphire
yellow = sapphire
Sapphire covers the largest number of colors. Gemologists and gem dealers will often deceivingly use the term "oriental" in referring to non-blue colored sapphires. For example, "oriental amethyst" is purple corundum, "oriental topaz" is yellow corundum, "oriental emerald" is green corundum, etc. Black-colored corundum is often called emery. Corundum-rich rocks are also called emery, or corundite.
Corundum (from left to right: 1.6 cm across, 2.3 cm across, 1.2 cm across, 1.3 cm across)
Ruby in amphibolite from Madagascar (Dwyer Mercer County District Library collection, Celina, Ohio, USA).
Ruby (large reddish mass at bottom center) in orthogneiss (consisting of whitish-grayish quartz, black biotite mica, dark red garnet, and bright red ruby) (4.5 cm across). Ruby is chromiferous corundum. This rock is a Neoarchean high-grade metamorphic rock from the Belomorian Terrane of the eastern Baltic Shield. It comes from Khit Island, Chupa Fjord District, far-northern Karelia, far-northwestern Russia.
Sapphires (largest specimen is 3.2 cm tall) - purple corundum.
Star sapphire (5.5 mm across) - polished gem-quality corundum sometimes reveals a shimmering 6-rayed star. This property is called asterism, or triple chatoyancy. This is produced by scattering of light by microscopic rutile (TiO2) inclusions arranged in three crystallographic directions at 120º from each other (Klein & Hurlbut, 1985). Star sapphires and star rubies are known.
Specimen owned by Mary Ellen St. John.
Rubies (5-6 mm in size each)
Ruby (red corundum) (public display, Cleveland Museum of Natural History, Cleveland, Ohio, USA).
Klein & Hurlbut (1985) - Manual of Mineralogy, 20th Edition. New York. John Wiley & Sons. 596 pp.