GO 340 Gemstones & Gemology

Emporia State University

www.emporia.edu/earthsci/amber/go340/corundum.htm

Ruby and Sapphire:
Varieties of Corundum

Corundum

Corundum, an aluminium oxide, Al2O3, is 9 on the hardness scale. It is in the trigonal subdivision of the hexagonal crystal system, which is reflected in the commonly found hexagonal prismatic crystal form. Corundum has no cleavage, but conchoidal or splintery fracture. It is transparent to translucent and has a vitreous to nearly adamantine luster. The specific gravity is high, 3.97-4.05. Corundum is doubly refractive, 1.762-1.778, with definite to strong pleochroism. Rutile, with an acicular habit, is a common inclusion that creates a chatoyancy, cat's eye effect, or asterism when the gem is oriented and cut properly. In a pure state corundum is colorless, but can also be red, blue, pink, orange, yellow, green, purple, and black. Gem quality red corundum is known as ruby, whereas any other color is termed sapphire.

Ruby occurs in metamorphic rocks, dolomitic marble and gneiss, whereas sapphire may be found in sedimentary limestones, metamorphic marble, and igneous basalt, pegmatites, or andesite dikes. Corundum is very rarely mined from primary deposits, but typically mined from secondary alluvial or placer deposits, where this resistant mineral becomes the sand and gravel of stream beds. Ruby and sapphire have been synthesized since the end of the 19th century for industrial and gem quality uses. Synthetic star ruby and sapphire have been marketed since 1947.


Sapphire from Urals, Russia, 10x8 cm. Image
taken from David Barthelmy's http://webmineral.com/specimens/picshow.php?id=287,
Mineralogy Database

Image taken from
Amethyst Mineral Galleries

Inclusions

Solid crystal inclusions found within corundum include apatite, calcite, chlorite, corundum, fluorite, graphite, olivine, pyrite, quartz, spinel, tourmaline, and zircon, to name a few (Hughes). Other inclusions include liquid/gas/solid filled cavities, that create "wispy" patterns, which may be confused with flux inclusions of synthetics; growth zoning, which are straight lines that follow the angular, hexagonal pattern reflecting the crystal system and form; and exsolved solid inclusions such as rutile needles and hematite-ilmenite plates. The exsolved rutile inclusions are called "silk" and are oriented in three directions at ~60/120° in the basal plane.

Treatments

Although gem treatment and enhancement probably go back several thousand years, modern methods for heat treating sapphire rough arrived by 1920, with ovens capable of reaching the necessary temperatures of over 1500° C (Hughes). Virtually all rubies and sapphires sold today are heat treated for color and clarity enhancement, with the exception of stones mined prior to 1975, that have not been subsequently treated (Hughes).

Image taken from
Amethyst Mineral Galleries

Image taken from
Tradeshop Inc

Geographic Sources

Some likely localities for corundum are given at mindat.org. Hughes wrote on, Myanmar (Burma) and India, along with specific pages on the Mogok ruby and Burmese sapphires. In addition to the more famous US sapphire sites in Montana, another location is found in Minnesota, according to the web authors at the University of Minnesota, Duluth.

Pricing

Colored stones, such as ruby and sapphire, lack a universally-accepted system of quality analysis needed to arrive at logical price structures. Hughes provides specific information on ruby and sapphire prices.

Image taken from
Jewelry Central.

Ruby

Ruby is the July birthstone and 15th and 40 anniversary stone. For specific birthstone information, visit: July birthstone: Ruby, jewelrycentral.com. Finely colored rubies are one of the most expensive of all gems, with the exception of rare colored diamonds. Because natural flawless ruby is virtually unheard of, synthetics are usually easy to spot! Visit Ramaura Cultured Ruby, at J.O. Crystal and Chatham created ruby for details. In addition to synthetics, common ruby imitators (The Jewelry Experts Bijoux Extraordinaire, Ltd.) are spinel, rubellite tourmalines and pyrope garnets. In addition, garnet may be misrepresented as Almandine ruby, Australian ruby, Bohemian ruby, and Cape ruby, while spinel may be called Balas ruby and Ruby spinel. Tourmaline may be referred to as Siberian ruby. It was not until 1800 that ruby was recognized as a separate mineral; prior to 1800 red spinel and red garnet were designated as ruby, from the Latin rubens which means red (Schumann, 1997, p. 82). Chromium is the coloring agent for ruby and the "pigeon's blood" is a desirable color, red with a overtone of blue.

Image taken from
The Mineral and Gemstone Kingdom
Famous rubies (Coyle Wholesale Jewelers), distinguished because of their size or extraordinary beauty, are located in various museums and private collections. For example, the 167-carat Edwardes Ruby, named in honor of Major-General Sir Herbert Benjamin Edwardes (1819-68) who served for the British in India, is located in the British Museum of Natural History. The Smithsonian has the 137-carat Rosser Reeves Ruby, a star ruby, while the American Museum of Natural History has the 100-carat Edith Haggin de Long Ruby. The famous Black Prince's Ruby and Timur Ruby, both part of the British Crown Jewels, are in fact red spinels. The Smithsonian has a most spectacular star ruby over 138 carats, the Rosser Reeves Ruby, as well as a recent acquisition of the largest faceted ruby in their collection, mineralsciences.si.edu/collections/newacquisitions/2004/carmenlucia.htm.
Indian Star Ruby
Image taken from
The Mineral and Gemstone Kingdom

Image taken from
The Mineral and Gemstone Kingdom

Sapphire

Sapphire is the September birthstone and for the 5th and 45th anniversary gemstone gifts. For specific birthstone information, September birthstone, Sapphire, jewelrycentral.com. Sapphire comes in all colors except red! It may be colorless, a deep blue, green, golden yellow, pink, reddish-orange, and violet. The most valuable color of sapphire is referred to as "corn-flower blue;" black, grey, or green overtones to this vivid blue will reduce overall value. Blue sapphire is not as expensive as diamond, emerald, and ruby, with the exception of the rare variety called Padparadscha, an orange-pink stone, and some pink stones.

Pink Sapphire
Image taken from
The Mineral and Gemstone Kingdom

Blue Star Sapphire
Image taken from
The Mineral
and Gemstone Kingdom
Needle-like, rutile inclusions in saphhire can be desirable when the gem is cut in a cabochon shape and asterism results! A six-rayed white star appears in bright lighting and the quality is judged by sharpness of star, symmetry of the rays, and the body color. The blue or black body color is host to the asterism in these images.

Famous sapphires include the 543-carat "Star of India," possibly the largest cut star sapphire, and the "Midnight Star," a black star sapphire, both housed at the American Museum of Natural History in New York City. The St. Edward's and Stuart sapphire are part of the British Crown Jewels and the Smithsonian owns the 330 carat "Star of Asia." See Coyle Wholesale Jewelers for more detail on famous sapphires.


Black Star Sapphire
Image taken from
The Mineral and Gemstone Kingdom
In times past, any blue gem material was called sapphire, such as lapis lazuli. Sappheiros is Greek for "blue." In order for sapphire to be that desired blue and other valuable colors, it has been estimated that 99.9% of all sapphire is heat treated to improve color and clarity. Visit Chatham Created Gems for details about created blue saphhire.
Image was taken from
University of Minnesota,
retrieved in 2003.
Many of the corundum references were to Richard W. Hughes, who has written extensively on his passion, ruby and sapphire. Additional excerpts from his book are online, as well as other gem topics such as jade.

Excerpts from a fascinating book, well worth reading:

  • Wise, R. (2003). Ruby and sapphire WWW URL: http://www.secretsofthegemtrade.com/chapter_22_o1.htm. From Secrets of the Gem Trade.
  • Wise, R. (2003). Burma ruby the boss is back WWW URL: http://www.secretsofthegemtrade.com/articles_3.htm. From Secrets of the Gem Trade.


    The material for this section came primarily from:

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    This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. For more information contact the course instructor, S. W. Aber, e-mail: saber@emporia.edu Thanks for visiting! Webpage created: November 5, 2000; last update: March 9, 2008.

    Copyright 1999-2008 Susan Ward Aber. All rights reserved.