CORUNDUM
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).
Reference
Cited:
Klein
& Hurlbut (1985) - Manual of Mineralogy, 20th Edition. New
York. John Wiley & Sons. 596 pp.