Hexagonal
Corundum (variety ruby) crystals in matrix.
A single tabular Corundum crystal showing growth hillocks on its pinacoidal
face.
Water worn, small pea-sized grains of Corundum from Montana. These were
recovered by a panning/screening process.
Corundum (variety Ruby) in Zoisite.
Corundum (variety Ruby) from Mysore,
Maharashtra State, India. Here alteration of the corundum is taking place
producing fuschite among other minerals.
CHEMISTRY Al2O3 Aluminum oxide
CRYSTALLOGRAPHY Hexagonal
CRYSTAL GROWTH AND HABITS Corundum forms hexagonal prisms or steep dipyramids, it may also be granular to massive.
COLOR AND OTHER OPTICAL PROPERTIES Corundum has a wide variation in colors, but most commonly corundum is clear, gray, brown, pink to red, blue, green and yellow. Red corundum is known as ruby, while all other colors are sapphires.
HARDNESS 9
SPECIFIC GRAVITY 3.9 - 4.1
LUSTER Adamantine to vitreous, sometimes pearly on parting plane
STREAK White
BREAKABILITY Commonly develops basal and prismatic parting, brittle
OCCURRENCE - Corundum is found in igneous rocks that are aluminum rich and silica poor (syenites and monzonites). It is also found in high grade aluminum-rich metamorphic rocks including marbles, schists and gneisses. Corundum is also found in placer deposits.
ASSOCIATED MINERALS Andesine, Oligoclase, Scapolite (in syenite), Spinel