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/gb4.htm

Gems in Brief

Many common and some not so common gems are translucent or opaque and 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 some common gems too.


Unmounted specimen is called Cheetah Serpentine
from Tanzania
Photo date 4/03,
© by S.W. Aber.
Serpentine is an aggregate of two minerals, antigorite and chrysotile. Both minerals are basic magnesium silicates (H4Mg3Si2O9). However, chrysotile also contains some iron in place of the magnesium within its chemical structure. These two aggregate minerals possess slightly different properties, most noticeably in fracture and cleavage. Chrysotile possesses a fibrous nature that creates an uneven, splintery fracture and no cleavage planes. Conversely, antigorite has a monoclinic crystalline structure that creates a micaceous habit. Therefore, antigorite has perfect cleavage in one direction and possesses a conchoidal fracture. The hardness for serpentine has a fairly wide range of 2.5 to 5.5 due to differing proportions of its two aggregates, though its fibrous nature ensures a high tenacity. It can also range in color from a dark, blackish-green to a yellowish-green (though the color is often very spotty) and possesses a waxy, silky luster. Serpentine has a specific gravity of 2.5 to 2.6 and can range from translucent to opaque in light transmission. Higher antigorite content produces more translucent stones. Due to its low hardness, serpentine is generally used for decorative and ornamental pieces. Also, certain varieties that are extremely fibrous are referred to as asbestos to be used for industrial applications, such as fireproof fabrics and brake linings.

Serpentine minerals originate from the rock serpentinite, usually formed from peridotite by hydrothermal metamorphic processes, and can come in a variety of forms that are defined by appearance and form. Bastite is a silky shiny variety with a prismatic crystal shape. Bowenite is an apple-green variety that often is interspersed with light spots. Connemara is a green rock that occurs due to the intergrowth of serpentine with marble. Verd-antique serpentine is a green rock interspersed with white calcite or dolomite veins. Williamsite is a translucent oil-green variety of serpentine that is often found with black inclusions. Fine williamsite samples are found locally (for U.S.) near Rock Springs, Pennsylvania. Serpentinite is found in numerous places in North America (Quebec, Colorado, Arizona, New Jersey, Pennsylvania, California) as well as in Afghanistan, China, and New Zealand. (J. Archuleta)


Photo date 4/02,
© by S.W. Aber.
Shell or Stone Cameo is a name for a carved figure that is raised and layered from shell or stone. The stone can be agate, onyx, sard onyx, carnelian onyx, or niccolo. To create a cameo, a piece of the layered stone is cut out or broken out and the bottom layer is dyed a dark color and the top layer is left white. The top, white layer is then carved, usually into a woman's head historically, but more recently, the carving can be whatever the artist desires. (J. Berg)

Photo date 4/02,
© by S.W. Aber.
Smoky quartz is silicon dioxide or SiO2, which is the same formula for clear quartz. Smoky quartz, as the name might imply, occurs in a pale brown to black color. The hardness is 7 and the specific gravity is 2.65. There is no cleavage but on rare occasions it can have indistinct rhombohedral parting. The crystals are hexagonal and usually prismatic crystals, with striations crosswise (unlike other minerals, such as emerald, that are striated lengthwise). Also, it can occur with doubly terminated rhombohedrons. Smoky quartz can be found in pegmatites in the region surrounding the Pikes Peak, Colorado, in the U.S., as well as Brazil. Smoky quartz is faceted into jewelry just as the yellow and purple varieties of quartz called citrine and amethyst (Chesterman, 1979, p. 502-504). Smoky quartz is popular as an ornamental stone that is carved into spheres, pyramids, obelisks, eggs, figurines and ornate statues. Smoky quartz, a variety itself of quartz, has a few varieties of its own:
  • Cairngorm is a variety that comes from the Cairngorm Mountains in Scotland.
  • Morion is a very dark black opaque variety of smoky quartz.
  • Coon tail quartz is a smoky quartz with an alternating black and gray banding.

The color of smoky quartz is variable from brown to black and sometimes smoky gray colored specimens are included as smoky quartz. The cause of the color of smoky quartz is in question but it is almost certainly related to the amount of exposure to radiation that the stone has undergone. Natural smoky quartz often occurs in granitic rocks which have a small but persistent amount of radioactivity. Most smoky quartz that makes its way to rock shops and to some gem cutters has been artificially irradiated to produce a dark black color. (C. Harris)

References:


Photo date 2/02,
© by Berg and Dougan.

Visit the sodalite stamp
page by Richard Busch!

Sodalite is a white or blue gray mineral with a hardness of 5.5 to 6. The mineral is transparent to opaque and has a white streak. Sodalite may sometimes be confused with azurite because of the same basic blue color. The fracture of sodalite is uneven or conchoidal and the cleavage is indistinct. Only the blue tones of sodalite are used in jewelry and they sometimes have a violet tint. Sodalite can be found in Brazil, Greenland, India, Canada, Nambia, Russia, and Montana. It is usually only used as an ornamental stone. (J. Sielert)

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

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Faceted sphalerite is from Spain
Photo date 4/03,
© by S.W. Aber.
The mineral name sphalerite is of Greek origin and means deceitful. This is due to the fact that it is used as an ore. It has a greasy or adamantine luster as does diamond. The dispersion factor is three times as high as diamond! What an interesting fact... Sphalerite is cut and faceted into jewelry for rings, necklaces and bracelets, but its low hardness keeps it from being a commonly requested gemstone.

Sphalerite, a zinc sulfide, can be reddish (Ruby Jack), greenish, yellow, colorless, and in its most common nongem color, brown. The streak is a pale yellow and the sulfur smell is evident. The hardness is 3.5-4 and it displays perfect 6 directional cleavage. Also, sphalerite has an uneven fracture and brittle tenacity. The specific gravity is high, at 3.90-4.10. Sphalerite is in the isometric crystal system and can be transparent or translucent.

Sphalerite is mined for the zinc. Economically important deposits are found in Canada, Mexico, Namibia, Spain, Wisconsin, and Zaire. Sphalerite used to mined in southeast Kansas, an region called the tri-state area. (J. Bray)

References:
The Audubon Society Field Guide to North American Rocks and Minerals, Charles Chesterman; Random House of Canada Limited, Toronto 1978.

Gemstones of the World, Walter Schumann; Sterling Publishing Co., Inc. New York 1999


Photo date 4/03,
© by S.W. Aber.
Spinel is a group of various colored minerals that are transparent to opaque in nature. The color ranges from red, orange, yellow, brown, blue, violet, purple, green to black. Spinel is an oxide of magnesium and aluminum, with iron, chromium , vanadium, cobalt, and zinc as coloring agents. It has a streak of white, hardness of 8, and density of 3.54 to 3.63. Its cleavage is indistinct and fracture, conchoidal and uneven. Spinel has a cubic crystal system, with the crystals being found as twins, octahedrons, and dodecahedrons.

Spinel was only classified as a separate mineral 150 years ago; before that it was classified as ruby. Two of the rubies in the English crown jewels, the “Black Prince's Ruby” and the “Timur Ruby” are really spinel.

Spinel is used as a substitute for many other gems, since the color is so varied and it can be found in great abundance. It is also less expensive than many of the other minerals. Spinel can be found in placer deposits with ruby and sapphire, mainly in Myanmar (formally Burma), Cambodia, and Sri Lanka. In the United States, it can be found in New Jersey.

Synthetic spinel have been available since the 1920's. It is used to imitate many gems, however the synthetic stones tend to be more brilliantly colored. Colorless spinel only occurs in synthetic. (K. Gaines)

References:
Schuman, Walter. Gemstones of the World. Sterling Publishing Co., Inc. 1999
Desautels, Paul E., The Gem Collection. Smithsonian Institute Press, 1979


Photo date 4/03,
© by S.W. Aber.
Staurolite - The transparent to opaque staurolite has a vitreous luster and comes in colors from reddish brown to black. One major interesting feature of staurolite is the interpenetration twinning, which can be seen in about 35 percent of the specimens. The crystals form in two twin types, x-shaped and +-shaped. Staurolite exhibits a hardness of 7-7.5 and a density of 3.7 to 3.8. Staurolite is in the monoclinic crystal system. Pleochroism can be seen, although colors and depth vary at different angles, from a strong yellow to yellow-red to red. Specimens are found associated with other metamorphic minerals such as almandine garnet, kyanite, and mica minerals. This cross form of this silicate mineral is associated with Christianity and considered by some to be good luck. Other names have been attributed to staurolite such as "Fairy Stone" or "Fairy Cross". (P. Harley)

References
Schumann, W. 1997. Gemstones of the world.
http://www.galleries.com/Staurolite.

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Photo date 3/02,
© by S.W. Aber.
Sugilite is a violet colored rock that is translucent to opaque. Sugilite has a hardness of 6-6.5, and a density between 2.76 and 2.80. This rock has a resinous luster and belongs to the hexagonal crystal system. Sugilite also has a refractive index of 1.607-1.611. Sugilite was formerly erroneously offered as Sogdianite. (J. Sielert)

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


Photo date 2/02,
© by Berg and Dougan.

Visit the sunstone stamp
page by Richard Busch!

Sunstone has been a favorite of gemstone collectors for many years. It forms in the basalt magma fields of southern Oregon and reaches the surfaces in magma flows. It has a Mohs hardness rating of 6-6 ˝. It is a labradorite that co-crystallizes with copper as it grows. The unusual "schiller" is created from the mixing with these tiny platelets of copper. This effect is reddish-orange in color with a golden iridescence or "aventurescence" flashing out at some angles. Additionally, light interference on hematite or goethite plates can also result in green or blue colors for this aventurine feldspar (Schumann, 1997, p. 166). (P. Mura and S. Gering)

Photo date 2/02,
© by Berg and Dougan.
Talc is a transparent to opaque mineral with a hardness of 1. The density is 2.55-2.80 and it appears in colors such as: gray-green, pearl-white, blue-green, and yellow. Talc is monoclinic and has a greasy luster as well as a greasy feel. Dense aggregates of talc are called soapstone and steatite and can be carved or worked into ornamental objects. It has even been used for a kitchen sink. (J. Sielert)

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


Photo date 2/02,
© by Berg and Dougan.
Tiger Eye is an opaque gold-yellow/brown species of quartz showing chatoyancy. It has a hardness of 6.5-7 and streaks yellow-brown. It has no cleavage but shows fibrous fracture. Tiger eye comes from the oxidation of iron in hawk's eye (which came from the replacement of crocidolite). It is cut in cabochon to best display its chatoyancy. (L. Flax)

Photo date 3/02,
© by S.W. Aber.
Thulite is a dense opaque pink zoisite variety, named after the legendary island Thule. Deposits of this mineral can be found in Western Australia, Namibia, and North Carolina. It is used as a gemstone cut en cabochon or as an ornamental stone. The chemical composition is Ca2Al3Si3O12(OH). Notable physical properties include: 6-6.5 hardness, vitreous luster, pink color, good cleavage, 3.2 to 3.4 specific gravity, uneven to conchoidal fracture. It belongs to the orthorhombic crystal system and fluoresces. (K. Barnett)

For additional information on thulite, a pink zoisite, visit stoneplus.cst.cmich.edu/zoisite.htm , R. V. Dietrich (2002). GEMROCKS: Ornamental & Curio Stones.


Photo date 4/02,
© by S.W. Aber.
Turquoise with a pure blue color is rare; mostly it is interspersed with brown, dark gray, or black veins of other minerals or the host rock. Stones interspersed with other minerals are called turquoise matrix. Turquoise can also be intergrown with malachite and chrysocolla. It has a waxy luster and white streak. Turquoise has a hardness between 5-6 and specific gravity of 2.31-2.84. It has no cleavage and breaks with a conchoidal to uneven fracture. Turquoise is in the triclinic crystal system and is translucent to opaque. Most of the so-called turquoise found in the U.S contains Fe and is thus really a mixture with chalcosiderite. Iron imparts a greenish color. Turquoise can be confused with amazonite, chrysocolla, odontolite, and serpentine. Notable localities for turquoise include: Iran, Afghanistan, Argentina, Australia, Brazil, China, Israel, Mexico, Tanzania, and U.S. (K. Barnett)

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


Photo date 2/02,
© by Berg and Dougan.
Ulexite is white with a silky luster and white streak. It is named after a German chemist and has a hardness of 2 - 2.5. It is also called TV stone because a piece of writing placed underneath the stone appears on the surface of the stone. When cut in cabochon it can show chatoyancy and a cat's-eye effect. Deposits are found in Argentina, Chile, Canada, Kazakhistan, Peru, Russia, and the U.S. It has perfect cleavage. (S. Kelley)

Photo date 4/02,
© by S.W. Aber.
Unakite is a variety of granite with green epidote and patches of reddish orthoclase. It is composed of feldspar, epidote, and quartz. Unakite has a hardness of 6 to 7 and a specific gravity of 2.86 to 3.2. It is believed to stimulate movement after healing and to balance emotions to higher forces of spirituality. It can be cut en cabochon or carved. It was named for the Unaka Mountains in Tennessee, but it is also found in Africa. (J. Berg)

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


Photo date 4/02,
© by S.W. Aber.
Victoria Stone is reconstructed material that comes in a variety of colors and shades of green, blue, pink, purple, among others. The material is reportedly created from quartz, calcite, fluorspar, magnasite, and/or feldspar, that is fused together creating a new substance similar in composition to amphibole minerals, such as nephrite jade. It is similar to nephrite jade in that it is compact and has a tough tenacity. The fibers have recrystallized in fan shaped clusters and when cut the material has a silky chatoyant character. The matial has a hardness of 6, specific gravity of 3.02, and refractive index of 1.62. (S. W. Aber)

Photo date 4/03,
© by S.W. Aber.
Zircon :
-Color: Colorless, yellow, brown, orange, red, violet, blue, green
-Luster: Adamantine
-Streak: White
-Hardness: 6.5-7.5
-Density: 3.93-4.73
-Cleavage: Indistinct
-Fracture: Conchoidal, very brittle
-Crystal System: Tetragonal
-Crystal Habits: Dipyramidal and prismatic
-Transparency: Transparent to translucent
-Luster: Vitreous to brilliant sheen
-Notable Occurrences: Seiland, Norway; Pakistan; Russia; Bancroft and Sudbury, Ontario, Canada and New Jersey and Colorado, USA
-Best Field Indicators: Crystal habit, hardness, luster and density

The name zircon was probably derived from the Persian language meaning golden colored. Zircons possess such a brilliant luster and fire that they are sometimes mistaken for diamonds. Because of these qualities the zircon is valued as a very acceptable and affordable alternative to the diamond. Zircons sometimes do not get the respect that they deserve, because those outside the gem world often hear the name zircon and relate it to the man made diamond alternative cubic zirconia or CZ. Zircons are natural and because of their beauty can stand on there own in the gem world.

References:
Amethyst Galleries, Inc. Copyright 1995, 1996, The Mineral Zircon,
http://www.galleries.com/Zircon. Site accessed 2-6-04.
Schumann, Walter, 1997, Sterling Publishing Co., Inc. New York, Gemstones of the World, Revised & Expanded Edition.

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Return to GO 340 Syllabus or go on to visit more gemstones...

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.