Corundum, Rutile Locality: Emmelberg, Üdersdorf, Daun, Eifel, Rhineland-Palatinate, Germany Image width: 4 mm Copyright © Fred Kruijen

Chemical Formula: Al2O3
Locality: Tchainit and Yakutia, Russia.
Name Origin: Probably derived from the Sanskrit, kuruvinda, meaning “ruby.”

Corundum is a crystalline form of aluminium oxide (Al2O3) typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is one of the naturally transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red and padparadscha if pink-orange. All other colors are called sapphire, e.g., “green sapphire” for a green specimen.

The name “corundum” is derived from the Tamil word Kuruvindam or Sanskrit word Kuruvinda meaning ruby.Because of corundum’s hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every 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, and the mix is less abrasive, with an average Mohs hardness of 8.0.In addition to its hardness, corundum is unusual for its density of 4.02 g/cm3, which is very high for a transparent mineral composed of the low-atomic mass elements aluminium and oxygen.

Physical Properties

Cleavage: None
Color: Blue, Red, Yellow, Brown, Gray.
Density: 4 – 4.1, Average = 4.05
Diaphaneity: Transparent to translucent
Fracture: Tough – Difficult to break apart as shown by fibrous minerals and most metals.
Hardness: 9 – Corundum
Luminescence: Sometimes Fluorescent, Long UV=red.
Luster: Vitreous (Glassy)
Magnetism: Nonmagnetic
Streak: none

Photos :

Corundum var. Ruby – Ambahatraso- Hiosy – Madagascar Specimen weight:60 gr. Crystal size:1,2 cm tall Overall size: 42mm x 30 mm x 38 mm
Corundum, Hematite, Sanidine Locality: Wannenköpfe, Ochtendung, Polch, Eifel, Rhineland-Palatinate, Germany 0.6mm high Copyright © Fred Kruijen
Corundum, Pseudobrookite Locality: Emmelberg, Üdersdorf, Daun, Eifel, Rhineland-Palatinate, Germany Picture width 3 mm. Copyright © Stephan Wolfsried
Corundum (Var: Sapphire) Locality: Mogok Township, Pyin-Oo-Lwin District, Mandalay Division, Burma (Myanmar) Dimensions: 2.4 cm x 1 cm x 1 cm Copyright © Danny Jones