Corundum





Authors: Melissa Gutierrez, Julissa Flores-Lopez, Lexi Branson

Mineral Name: Corundum

Chemical Composition: Al₂O₃ (1)

Color: The color of corundum varies; different colors appear in the presence of various metals that are impurities in the chemical composition. For instance, chromium creates red (ruby) and iron and titanium create blue (sapphire). If no metal impurities are present, color ranges from gray to brown or colorless if it is pure.

Streak: The streak is colorless

Hardness: Corundum is harder than porcelain, so it would be at least an 8.5-9 on Moh’s Hardness Scale.

Cleavage/Fracture: Corundum has no cleavage but can have rhombohedral or basal parting, which are types of fracture. (1)

Rhombohedral parting is when a crystal breaks along three axial planes of equal length because of unconformities in the crystal, like twinning and chemical alteration. Basal parting is when “peeling” occurs from the base of the mineral. (4)

Crystal Form: Typically corundum forms a hexagonal shape prism, but with smooth and rounded edges. They can also appear as “bipyramidal hexagons”, meaning they are more of a typical diamond shape that may be more pointed (tapered) and thin at the ends, and wider in the center. (2)

Luster: Ranges from vitreous to adamantine. (2)

Special Features: One of the special features of corundum is it’s hardness, which is rated at a 9 on Mohs hardness scale, which is the 2nd hardest mineral on the scale. Due to the crystal form and supreme hardness of this mineral, corundum is great for cutting other gemstones (sapphire and ruby). Corundum is also known for its’ high specific gravity and its very distinctive crystal structure.

Varieties: The mineral, when cut into gemstones, creates the gems that are known as ruby and sapphire. These gemstones can get their color through artificial means or can be deepened through heat treatment. Other varieties of corundum include but are not limited to: emery, padparadscha, star ruby, star sapphire, ultralite, oriental topaz, trapiche ruby. (4)

Mineral Group: Corundum is an oxide mineral and is a member of the hematite group. An oxide mineral is one that contains an oxide anion (O2-) is bonded with one or more metal ions. Corundum belongs in the hematite group of oxide minerals because of its tabular crystal habits, lack of cleavage and hexagonal crystal system. (11)

Environment: Generally speaking, corundum is best formed in environments of low silica and high aluminum content. (7) The best sapphire and ruby deposits can be found where the gems weathered from flows of basalt and are transported into soils and sediments. The best quality of rubies and sapphire gems form within marble along the edges of magma bodies below the surface of the Earth. Corundum is also within metamorphic rocks where aluminum rich shales/bauxites are introduced to contact metamorphism. Regional metamorphism produces gneiss, marble, and schist and these rocks can contain corundum as well (1).

Associated Rock types: Some rocks associated with corundum include: mica schist, gneiss, and also some marbles found in metamorphic terranes. Corundum also is found in syenite (a low silica content intrusive igneous rock) and nepheline syenite intrusive rocks.

Occurrence in North America: Corundum was once mined from deposits in the dunites of North Carolina, and from the nepheline syenites of Craigmont, Ontario. Another well known occurrence of corundum is within the Burgess Mine, Bancroft, Hastings Co. located in Ontario. Other sightings of corundum include: deep blue sapphire crystals found in the Yogo Gulch. Waterworm sapphire stones can be found in the Missouri River (near Helena, MT). Elongated corundum crystals were spotted in San Bernardino, California throughout the Cascade Canyon. A place known for ruby sightings is the Cowee Creek District, Macon Co. located in North Carolina. Emery can be found near Peekskill, Westchester Co. in New York and also in Chester, Hampden Co. in Massachusetts. (4)

Economic Uses: Corundum is used as a precious stone and can be commonly found in jewelry as sapphire and rubies. Like many other precious stones, corundum is used to create rubies and sapphires through artificial processes as well. Heat is used to change the mineral inclusion structures inside of these gems. This change in what is referred to as “silk” composition in the gemstone changes the transparency of the stones, producing a more transparent and beautiful blue sapphire or red ruby. These precious stones have been found to be used as money and as ornaments by ancient Romans and Greeks. (12)

Industrial Uses: Due to its hardness, corundum is commonly used for grinding, polishing, in sand papers, and other cutting tools. (1) In order to be used this way mineral samples are crushed into finer particles. The problem with using natural samples is the inconsistency in hardness, so synthetic corundum is used instead. Synthetic corundum is made through the Verneuil process developed by French chemist, Auguste Verneuil. Part of the process is creating a boule, “a mass alumina that has the same physical and chemical characteristics as corundum.”(13) The Synthetic gem is composed of highly purified alumina that goes through a fine sieve that is then tapped by a mechanical hammer put into an enclosed chamber that is then heated at a temperature above boiling until the boule is the desired form and size. (13)

First Notable Identification: The corundum gems, ruby and sapphire, have been recognized since prehistoric times. Corundum has been found as an abrasive grit in prehistoric artifacts from China’s Neolithic period (c. 4000–3500 BC) in Axes from Liangzhu and San Xingcun cultures. (9) The first notable discovery of Corundum in large quantities, in order to be useful as an abrasive, was in the Grecian Archipelago. Up until 1847 almost all abrasive purpose corundum was mined from there.(8) There is no definite time period of when corundum was first identified or used.

How We Identified It: When you think about corundum you think rubies and sapphires so one thinks deep red and a deep blue. But if untreated, like common hand samples, they are easier to identify by shape and only partially by hue. Corundum is an elongated mineral with a hexagonal shape at its natural breaking point. It has a reddish hue to it to it despite other impurities covering it in a rusty or whitish grey color deposits.

Don’t Confuse It With: Spinel is commonly confused with Ruby and Sapphire because this other mineral is also often found in the similar hues of corundums bright reds and deep blues. Spinel is found misnamed in “crown jewels” like the “Timur Ruby” a 352.5-carat bright red spinel that is in a necklace that was made for Queen Victoria in 1853.(10) It was presented by the East India Company with a collection of other stones. They are commonly confused because for thousands of years gemstone traders thought that all gems that were red were rubies and all blue gemstones were sapphires.

Bibliography:

  1. “Corundum.” Geology, geology.com

  2. “Corundum: The Mineral Corundum, Sapphire, Ruby Info & Pictures”. Minerals.net.

  3. “Rhombohedral Crystal System.” The Free Dictionary, Farlex.

  4. “Cleavage: Mineral Properties - The Mineral and Gemstone Kingdom”. Minerals.net.

  5. "Corundum." Corundum: Corundum Mineral Information and Data. N.p., n.d. Web. 12 Mar. 2018.

  6. "Corundum." Wikipedia. Wikimedia Foundation, 12 Mar. 2018. Web. 12 Mar. 2018.

  7. "Corundum." Earth Sciences Museum. N.p., 26 June 2014. Web. 12 Mar. 2018.

  8. Pratt, Joseph H. “Corundum and Its Occurence and Distribution in the United States.” 1906. PDF file.

  9. P. J. Lu; N. Yao; J. F. So; G. E. Harlow; J. F. Lu; G. F. Wang; P. M. Chaikin. “The Earliest Use of Corundum and Diamond, in Prehistoric China.” Abstract. Archaeometry. 47.1 (2005): 1-12. Wiley Online Library. Web. 12 March 2018.

  10. King, Hobart M. “Spinel A Gemstone That Was Confused with Ruby and Sapphire for over 1000 Years.” Geology.com. Web. 12 March 2018.

  11. Britannica, The Editors of Encyclopaedia. "Oxide Mineral." Encyclopædia Britannica. Encyclopædia Britannica, Inc., 21 Dec. 2012. Web. 12 Mar. 2018.

  12. Gemstone Treatments - Heating & Irradiation. N.p., n.d. Web. 12 Mar. 2018.

  13. Britannica, The Editors of Encyclopaedia. "Verneuil Process." Encyclopædia Britannica. Encyclopædia Britannica, Inc., 22 Oct. 2013. Web. 12 Mar. 2018.