Aquamarine, Brazil. Photographed by Chip Clark copyright. www.nmnh.si.edu

Precious Gemstones

Diamond - The diamond is the hardest known naturally occuring substance. It is composed only of carbon. Diamonds were first found in central India. Large amounts of diamonds were are also found in Brazil and South America. The Brazilian diamonds are purer than the African diamonds. Diamonds are generally colorless although some are tinted blue, pink, yellow, and other colors.

Ruby- Rubies are one of two precious gemstones consisting of Corundrum. It is the second hardest gemstone. Rubies are various shades of red and the most precious shade is a red with a slight tint of purple ound in Burma. Afghanistan has famous ruby mines as will including one called "Amir's mine".

Sapphire- The sapphire is the second precious gemstone from Corundrum. It's standard color is blue and the color is not always uniform within the stone itself. Large quantaties of sapphires have been extracted from the Himalayan mountains. Both Sapphires and Rubies are easy to produce artificially because of the properties of Corundrum.

Emerald- The Emerald is made from Beryl. It's greenish hue comes from silicate of aluminum and beryllium. Emeralds are always a six sided prism. They are softer than the above gemstones. Historically Emeralds have been known to be found in Egypt and Russia. Presently though they can be found in Columbia.

Aquamarine- The Aquamarine is also formed from Beryl. It is generally the color of sea water (either bluish green or greenish blue). Aquamarienes found in Brazil are highly unusual due to their very blue color.

Topaz- The most common color of Topaz is yellow. They can be many, many colors though, including light green, light blue, red, or colorless. The finest colored Topaz comes from Brazil. They have a rhombic prism and cleavage.

(Kraus, 1947)

Birthstones

A common use for precious and semi-precious gemstones is alloting each a month and calling them birthstones.

(USGS, 1997).

Amentrine. Photographed by Chip Clark. copyright www.nmnh.si.edu

Diamond Star of Sierra Leone. Photographed by Chip Clark. copyright www.nmnh.si.edu

Hooker Emerald. Photographed by Chip Clark.copyright www.nmnh.si.edu

Fire Opals. Photographed by Chip Clark,.copyright www.nmnh.si.edu

Hope Diamond. Photographed by Dane Penland.copyright www.nmnh.si.edu

turquoise. copyright www.theimage.com

topaz. copyright www.theimage.com

Literature Cited

Whitlock, Herbert. The Story of the Gems, Lee Furman. 1936: New York

Kraus, Edward. Slawson, Chester. Gems and Gem Materials, McGraw-Hill Company, Inc. 1947: New York

Marshak, Stephen. Essentials of Geology. W.W. Norton & Company. New York: 2004

USGS. www.pubs.usgs.gov last modified 6-18-97

National Museum of Natural History. Smithsonian. March 2005. www.nmnh.si.edu

Amythst Galleries. "Gemstones" copyright 2004. www.mineral.galleries.com

What Are They

Precious Gemstones can be either a mineral, stone, or organic matter. Precious gemstones are generally considered to be the diamond, ruby, sapphire, emerald, aquamarine, topaz, and opal. All other gemstonesare semi-precious. What distinguishes a precious stone from a semi-precious stone is it's availability and demand. Stones that are readily available and common or do not have a high demand are semi-precious whereas stones that are rare and have a high demand are labeled as being precious.

How Are They Formed

Gemstones form many different ways.

-They can form through solutions like the Opal. The solution can evaporate, change in temperature and/or pressure, or undergo a chemical reactoin with other solutions, gases, or solids.

-Gemstones can also form by fusion like the Diamond, emerald, and sapphire.

-Magma cools and crystalizes forming igneous rocks. Gemstones are usually the last things to crystallize. Gemstones formed after the solidification of a melt are frequently found in pegmatites.

-Metamorphism is a process that's end result can be precious gemstones. The emerald forms this way. This occurs by a change in heat, pressure, or moisture. Contact metamorphism caused by head entering the rock body, produces emeralds.

-Another method of formation is an organic process. Amber, Jet, and Pearls are formed through an organic process. These organic processes include reactions of organisms with solutions or solutions with organic material.

Gemstones can occur in all kinds of environments. Each environments tends to have certain kinds of minerals that form there, but several types of gemstones can occur in multiple environments. Most gemstones form in igneous rocks, though they also occur in metamorphic and sedimentary rocks. They form in veins and cavities in the rock, not in the typical mineral deposits.

(Marshak,2004) (Kraus, 1947)

Identifying Gemstones

Gemstones are classified by their:

- hardness: based on how easy/hard it is to scratch the gem; depends on the bonds in the lattice

-specific gravity: the density of a gem

- color: how the gem interacts with light

- chemical composition: minerals that make up the gem stone

- optical character: the geometry of the gem, its crystal faces

(Whitlock 1936)

Classifying Gemstones

- Elements class: diamonds

-Oxides class: corundrum

-Carbonate

-Phosphate Class: Turqoise

-Silicate: Beryl

-Mineraloids: opal

(Amythst Galleries 2004)

Link to other Student Webpages for 2005 Earlham Physical Geology

Copyright © 2004 Earlham College. Revised April 11, 2005 . Send corrections or comments to parkero@earlham.edu