GO 340 Gemstones & Gemology
ES 567 Gemstones of the World
Dr. Susan Ward Aber, Geologist & Gemologist
Emporia State University
Emporia, Kansas USA

academic.emporia.edu/abersusa/go340/gb1.htm

Gems in Brief

Many common and some not so common gems are translucent or opaque, fragile or unstable. Therefore visual observations, with and without magnification, are used for identification because common tests for refraction and specific gravity may be inconclusive or destructive. Images and short descriptions below were prepared primarily by students to catalog some of the lesser known gemstones, as well as a few better known gems.


Photo date 2/03, © by S.W. Aber.
Emerald is the green variety of beryl with a chemical composition of Be3Al2Si6 O18. This beryllium aluminium silicate frequently has some sodium, lithium, and cesium included in the mineral ( Chesterman, 1979, p.560). The luster is vitreous and beryl has a colorless streak. The hardness is 7.5-8 and specific gravity ranges from 2.66 to 2.92. Beryl's fracture is uneven to conchoidal and the cleavage is indistinct in one direction. Gem quality stones are transparent to translucent. The crystals are hexagonal and are usually six sided prisms that are striated lengthwise. The natural emerald is noted for its deep green color and the presence of inclusions verify its natural origin.
Colors for beryl include blue-greenish blue (aquamarine), yellow (golden beryl), light yellow green (heliodor), red (bixbite), pink or peach (morganite), colorless (goshenite), as well as the bright green emerald, which is considered the most valued of these varieties. Beryl develops in pegmatites and certain metamorphic rocks. Fine emeralds have velvety body appearance and the value is in the even distribution of color. Beryl can also have a pale green variety that is not gem quality and this mineral occurs with scheelite in a pegmatite near Oreana, Pershing Co. Nevada as well as in North Carolina, Colorado and California (Chesterman, 1979, p. 560-563). (C. Harris)
Reference:
Chesterman, C. (1979). The Audubon society field guide to North American rocks and minerals. NY: Knopf.

Image taken from:
http://www.theimage.com/mineral/dioptase/7.htm
Dioptase is a green gem and the name means to view through in Greek. It has perfect cleavage, conchoidal fracture and is brittle. The hardness is 5, with a density of 3.28-3.35. This emerald green, blue-green gem can be confused with demantiod, diopside, fluorite, emerald, uvarovite, and verdelite. (M. Miles)

Reference:
Schumann, W. (1997). Gemstones of the world. NY: Sterling Publishing Co., Inc.

For more information visit,
http://www.webmineral.com/data/Dioptase.shtml
http://www.theimage.com/mineral/dioptase/index.htm


Photo date 2/02, © by Berg and Dougan.
Fiber optic material is synthetically made and meant to imitate chatoyancy and the cat's eye effect. This material is referred to as fiber eye and consists of tightly packed parallel glass fibers are transparent along their length. When the material is polished perpendicular to the fiber direction and with a cabachon cut the special phenomena, chatoyancy and a cat's eye effect, is evident. (S.W. Aber)

Photo date 2/02, © by Berg and Dougan.
Fire agate is opaque, limonite-bearing layered chalcedony with an opal-like play of color. It has a hardness of 6.5 to 7 and a specific gravity of 2.6 to 2.64. Fracture is uneven and it has no cleavage. The chemical composition is silicon dioxide. (K. Barnett)

For additional information on fire agate visit http://stoneplus.cst.cmich.edu/agate.htm, R. V. Dietrich (2002). GEMROCKS: Ornamental & Curio Stones.


Photo date 2/02, © by Berg and Dougan.
Fire opal is a variety of opal that is transparent to translucent and colored yellow-orange to red with a play of colors. Opal has a hardness of 5.5 to 6.5 and a specific gravity of 2.0 to 2.2. It has a vitreous or pearly luster. Fire opal can be found in lava flows in New Mexico. The word opal comes from the Sanskrit word upala meaning "precious stone." The ring shown at the left is orange common opal, whereas the faceted gem and rough specimen is fire opal. (J. Berg)

Visit the opal stamp by Richard Busch, Philatelic Mineralogy!

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Photo by S.W. Aber. Visit the fluorite stamp by Richard Busch!
Fluorite is a transparent mineral that comes in all colors including colorless. It is unique in that it often displays bicoloration or more than one color in a single crystal. Most fluorite fluoresces a strong blue-violet. Fluorescence was first recognized in fluorite. The luster is vitreous and when used as a gem, it can be cut en cabochon or faceted. There is perfect cleavage into distinctive octahedrons. The specific gravity is 3-3.25 and fluorite has a hardness of 4. The low hardness and perfect cleavage make it more durable when used in a necklace, earrings, or brooch, as opposed to a ring. (L. Flax)
For additional information on fluorite visit stoneplus.cst.cmich.edu/fluorite.htm , R. V. Dietrich (2002). GEMROCKS: Ornamental & Curio Stones.


Photo date 1/03, © by S.W. Aber.
Garnet is a transparent gem that includes many different varieties based on slightly different chemical compositions. The chemical make-up of the garnet can be Mg3Al2(SiO4)3, Fe3Al2(SiO4)3, and Mn3Al2(SiO4)3. It ranges in color, including red, red-brown, black, green, orange, purple, and yellow. Garnet varieties vary, not only in color, but also in value. Although the deep red color is desirable, if a garnet is too dark, it has less value. The method of cut influences the darkness of the color.
Of the garnet varieties, Almandine is a deep-red stone, which gets its color from iron. It is usually cut in to ovals since deeper cuts tend to make it appear too dark. Pyrope is the firey red garnet and tends to be large stones, though typically not as valuable. Pyrope gets its color from magnesium. A mix of almandine and pyrope creates "rhodolite". It has a reddish-purple color, and is one of the more expensive garnets. Spessarite is orange to orange-brown in color. It gets its color from Manganese. The value of spessarite is similar to rhodolite, and large stones are unusual. The other family of garnets includes the yellow-brown grossular variety from Sri-Lanka. There is a variety that is green in color, similar to the emerald. The color comes from chromium, and is used as an emerald substitute. The other three varieties are andradite, demantoid, and uvarovite. These range in color from black to green. Black garnets have very little value, whereas the green garnets are rare and valuable.
The most valuable garnet is the Idaho Star Garnet. The garnet is deep purple or plum in color with a 4 or 6-sided star floating on the surface. It is considered to be more valuable than both the star ruby and the star sapphire. Garnets have a hardness of 6.5 7.5, and an imperfect fracture. (K. Gaines)

Photo date 2/02, © by S.W. Aber.
Hawks eye is a pseudomorph of quartz after crocidolite. It is a blue-gray form of the more common tigers eye. The blue-gray color is from the crocidolite, which is a blue-gray asbestos mineral. It has a hardness of 6.5-7 and a specific gravity of 2.58-2.64. It is opaque with a silky luster. It has no cleavage, but a fibrous fracture. Hawks eye is sensitive to some acids. It is often found along with tigers eye in South Africa, Australia, Burma, India, Namibia, and the United States. It is commonly fashioned into cabochons which is a cut that maximizes the chatoyancy of the mineral. (A. Hess)

Photo date 2/02, © by Berg and Dougan.
Hematite is a metallic mineral with a high specific gravity of 4.9 to 5.3. It is in the hexagonal crystal system, though it is usually found massive or in granular masses. Hematite has a hardness of 5.5 to 6.5 and a dark reddish-brown streak; streak is one of the most distinguishing characteristics of this mineral (it is easily confused with magnetite). When polished, it has a beautiful shiny metallic luster over a dark steel-gray color; before polishing, it can be red, reddish brown, black. The name "hematite" is from the Greek word "haimatites", meaning "blood like," because of the red color of the powder. In the United States, it is found at Lake Superior. (J. Berg)
For additional information on hematite visit stoneplus.cst.cmich.edu/hematite.htm , R. V. Dietrich (2002). GEMROCKS: Ornamental & Curio Stones and hematite stamp by Richard Busch!

Photo date 3/02, © by S.W. Aber.
Howlite is a snow-white opaque gem, often with dispersed black or dark-brown veins. It has a Mohs hardness of 3-3 and a specific gravity of 2.45-2.58. It is a mineral classified in the monoclinic crystal system and not common. Howlite is easily dyed because it is porous and therefore used to imitate gems such as turquoise and lapis lazuli. Howlite was named after Henry How, the geologist from Nova Scotia who first discovered it; it is found in Canada (Nova Scotia, Newfoundland, New Brunswick), and the United States (California) (http://www.gemhut.com/howlite.htm). This particular rough came from California. White howlite can be dyed to imitate other stones such as turquoise and lapis lazuli, as shown by the earrings shown in the image. (P. Mura)

Photo date 3/02, © by S.W. Aber.
Iolite or cordierite is found in the colors blue, violet, and brown. Iolite has a hardness of 7-7.5. It has good cleavage, as well as conchoidal and uneven fracture, with a brittle tenacity. The streak of iolite is white and it can have a greasy or vitreous luster. Iolite with inclusions of hematite and goethite have a reddish aventurescence. Iolite is also called cordierite and dichroite. It can be found in Myanmar (Burma), Brazil, India, Madagascar, Sri Lanka and the United States. Iolite has a misleading trade name of Water Sapphire and is sometimes confused with benitoite, kyanite, sapphire, and tanzanite. Glass imitations are known for iolite. (J. Sielert)

Photo date 2/02, © by Berg and Dougan.
Ivory is a white to creamy colored organic substance or mineraloid. It has a hardness of 2-3 and low specific gravity of 1.7-2. The fracture is fibrous and it is opaque to translucent. Ivory can refer to the tusk or teeth of elephant, hippopotamus, narwhal, sea lion, and wild boar. There is a worldwide ban on trade in elephant ivory (since 1989) and US ban since 1994. (S.W. Aber)

Photo date 1/03, © by S.W. Aber.
Jadeite Jade has a chemical composition of NaAlSi2O6, a sodium aluminum silicate, often with calcium and iron. Jadeite is in the pyroxene group. It has a hardness of 6.5-7 and distinct, two directional cleavage at approximately 90 degrees. The specific gravity is 3.3-3.5 and it can have an uneven or splintery fracture. Jadeite jade is translucent to opaque with a tough tenacity. It falls in the monoclinic crystal system and crystals are rare. Good identifying characteristics are its toughness, distinct cleavage, and higher specific gravity and hardness as compared with nephrite jade. The mineral environment is metamorphic and it occurs with glaucophane in blue schist of regional metamorphic rocks.
The finest gem quality comes from Myanmar. Semitransparent to translucent varieties of jadeite are called jade. Colors are white to green, white with greenish spots (most common), and pale lavender is less common. Other colors include Emerald Imperial jade, silvery-white, and reddish brown. The value increases with transparency, color and freedom from flaws.
Jade is carve into beads, earrings, bracelets, and cabochons for rings and brooches, or carved into ornamental objects. Jade is also said to cure kidney disorders if applied to the side of the body. (J. Bray)
References: The Audubon Society Field guide to North American Rocks and Minerals, Charles Chesterman; 1978 Random House of Canada Limited, Toronto.
For additional information on jade and look alikes, such as vesuvianite, visit sites from R. V. Dietrich (2002),
GEMROCKS: Ornamental & Curio Stones:
  • stoneplus.cst.cmich.edu/jadeite.htm
  • stoneplus.cst.cmich.edu/jade.htm
  • stoneplus.cst.cmich.edu/vesuvianite.htm

  • Photo date 2/02, © by Berg and Dougan.
    The gemstone name jet comes from a river in Turkey. It is a bituminous coal that is able to be polished. The luster is velvety and waxy. It was used as special mourning jewelry by the aristocrats and royals, but also as rosaries, ornamental objects, and even cameos. It is shaped on a lathe. Deposits are found in a wide number of areas, such as, England, Germany, France, Poland, Spain, the United States (esp. Colorado, New Mexico, and Utah). This is an extremely soft gem with a Moh's hardness of 2 - 4, and a density of 1.19 - 1.35, showing a conchoidal fracture with no fluorescence. (P. Mura)
    For additional information on jet visit
  • coal stamp by Richard Busch!
  • stoneplus.cst.cmich.edu/jet.htm , R. V. Dietrich (2002). GEMROCKS: Ornamental & Curio Stones.
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    Gems in Brief A Gems in Brief B-C Gems in Brief D-J
    Gems in Brief K-N Gems in Brief O-R Gems in Brief S-Z

    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: esu.abersusie@gmail.com Thanks for visiting! Webpage created: 2002; last update: September 10, 2012.

    Copyright 1999-2012 Susan Ward Aber. All rights reserved.