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/gb.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.



Image taken from:
http://www.theimage.com/mineral/barite/4.htm, from The Image.
Barite, also called barytes and heavy spar, is best known for its high specific gravity of 4.43 to 4.46. It comes in many colors including white, gray, colorless, yellow, brown, red, green, and blue. The luster is vitreous or pearly and the streak is white. It has a hardness of 3 to 3.5 and has perfect cleavage in one direction. The crystal system that barite belongs to is orthorhombic. Barite can be found in British Columbia, Colorado, South Dakota, and California.
References
1. Chesterman, Charles W. National Audubon Society Field Guide to North American Rocks and Minerals. Orthoclase. Chanticleer Press, Inc. 1979. p. 508-510.
2. Schumann, Walter. Gemstones of the world. Feldspar Group: Amazonite. Sterling Publishing Co., Inc. 1995. p. 164.

Cut benitoite for Dr. Louderback.
Image taken from
http://www.mip.berkeley.edu/
geology/benitoite/Gemscans/cutben.html
.


Benitoite on natrolite.
Image taken from
http://www.mip.berkeley.edu/
geology/benitoite/Gemscans/238_22a.html
.

Benitoite is a rare transparent blue mineral found in San Benito County, California, USA and New Zealand. Benitoite is a barium titanium silicate in the hexagonal (trigonal) crystal system. It has poor cleavage and a hardness of 6-6.5. The luster is vitreous and specific gravity of 3.64-3.67. Although first discovered in 1906 by J. M. Couch and L. B. Hawkins, it was mistaken for sapphire. Nearly a year later it was identified as a new mineral by G. D. Louderback, a mineralogy professor at University of Southern California. Other rare and beautiful gems found in association with benitoite are, neptunite, joaquinite, and natrolite. The mineral is highly prized and is highly priced! The finest benitoite trades at $1000 per carat in 1998 and yearly production was anticipated between 50,000 and 100,000 carats (The Gemstone Forecaster Vol. 16(1), Part One). On February 1, 1999, the AZCO Mining Inc., a copper company, purchased the Benitoite mine in San Benito County, California (See this site for details: http://www.consrv.ca.gov/cgs/information/publications/cgs_notes/note_11/note_11.pdf. Benitoite became the state gem of California in 1985. (J. Inmon and S. Aber)
External links of interest include:
  • Veevaert, J. (2003). Benitoite. BenitoiteMine.com. WWW URL: http://www.benitoitemine.com/benitoite/benitoit.shtml. Excellent and inclusive!
  • Anthony, E. B. (no date). Let's talk gemstones. The New Mexico Facetor. WWW URL: http://www.attawaygems.com/NMFG/Lets_talk_gemstones_benitoite.htm . Retrieved 2-17-04
  • Attaway, N. (no date). The prez sez: On cutting benitioite. The New Mexico Facetor. WWW URL: http://www.attawaygems.com/NMFG/Prez_sez_cutting_benitoite.html. Retrieved 3/04.
  • Gem Hut (2004). Benitoite. WWW URL: http://www.gemhut.com/ben.htm. Retrieved 2-22-04. <
  • Mindat.org (no date). Benitoite WWW URL: http://webmineral.com/data/Benitoite.shtml. Retrieved 3/04.
  • Amethyst Galleries (2002). The mineral benitoite. WWW URL: http://mineral.galleries.com/minerals/silicate/benitoit/benitoit.htm. Retrieved 3/04.
  • Benitoite (1995). WWW URL: http://www.mip.berkeley.edu/geology/benitoite/.
  • Louderback, G. D. (1909). Benitoite, its paragenesis and mode of occurrence. University of California
    Publications Bulletin of the Department of Geology, 5
    (23), 331-380. WWW URL: http://www.mip.berkeley.edu/
    geology/benitoite/Louderback/1909_toc.html
    .

  • Photo date 2/02, © by S.W. Aber.
    Bloodstone is also called Heliotrope (Greek, sun-turner) in Europe. Sometimes called blood jasper in the trade. It is not a jasper but rather a dark-green chalcedony with red spots. Although the color is not consistent, the red spots are caused by iron oxide and the green color is from chlorite particles or hornblende needles. It has a hardness of 6.5 - 7, no cleavage, and a density of 2.58 - 2.64. It is composed primarily of silicon dioxide. It is often used as seals for men's rings and for ornamental objects. It is found in India, Australia, Brazil, China, and the United States. (S. Kelley and J. Willard)

    For additional information on bloodstone, a variety of chalcedoney, visit http://stoneplus.cst.cmich.edu/chalcedony.htm, R. V. Dietrich (2002). GEMROCKS: Ornamental & Curio Stones.


    Photo date 3/02, © by S.W. Aber.
    Visit the opal stamp by Richard Busch!
    Boulder opal has a dark base and play of color. It has a high density and is composed of a silica gel with water included, basically the composition of quartz. SiO2 *H2O. Water content can vary from 1-20% or more. Streak is white and hardness is between 5.5- 6.5. Opal fluorescences white or bluish, and brownish or greenish. Opal is heat and pressure sensitive. Specifically boulder opal is found within layers of coarse, hardened sandy clay. The thin seams can only be utilized by cementing them to a backing material creating a doublet or sandwiching the opal in the middle of three layers called a triplet. The backing material in a doublet or triplet is potch opal (without fire), obsidian, or a ceramic material with a black epoxy cement. In a triplet the thin opal slice is topped with a quartz cabochon which protects the opal from scratching. (K. Barnett)
    Return to the top.

    Photo date 2/03,
    © by S.W. Aber.
    Carnelian is a translucent to opaque variety of chalcedony. The orange to brownish red color comes from iron and the color can be enhanced by heating. The hardness is 6.5-7 and specific gravity is 2.58-2.64. Carnelian has no cleavage and uneven fracture. It has a dull to waxy luster and can be somewhat fibrous in appearance. It occurs in the monoclinic crystal system. Deposits are found in Brazil, India, and Uruguay. Many carnelians offered today are agates which are dyed and then heat treated. Natural carnelian shows a cloudy distribution of color. (J. Bray)

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


    Photo date 2/02, © by S.W. Aber.
    Cat's eye is a variety of chrysoberyl that has fine, parallel inclusions which cause a silver-white line reminiscent of a cat's pupil. Chrysoberyl has a hardness of 8.5, a white streak, and a density of 3.70-3.78. The chemical composition of chrysoberyl is BeAl2O4. Cat's eye is found in Sri Lanka, Brazil, China, India, and Zimbabwe. (A. Dougan)

    Specimen from Charo River, Russia. Photo date 4/03, © by S.W. Aber.
    Charoite is a hydrated potassium sodium calcium barium strontium silicate, K(Na, Ca)11(Ba, Sr)Si18O46(OH, F)-nH2O. Charoite is a rare and unusual inosilicate mineral. It has been found only one place in the world, along the Chary River at Alden in Siberia, Russia. The color is white, lavender, lilac, violet and/or purple; the streak is a pale purple. The luster is vitreous to pearly and it is transparent to translucent. The crystal system is monoclinic and the crystal habit is fibrous, with interlocking crystal masses. Charoite is has a massive habit. It has no cleavage, but the fracture is conchoidal. The hardness is 5-6 and specific gravity average at 2.5-2.8.
    Charoite forms from alterations in limestone caused by the presence of an alkali-rich nephline syenite intrusion. The combination of hot circulating chemicals and the heat and pressure of an intrusion, creates new minerals like charoite. Charoite is used as an ornamental stone and as a gemstone. The swirling patterns are interlocking crystals make for an interesting gemstone. This stone would be more popular if it did not have a synthetic look to it even though it is naturally occurring. (C. Harris)

    Also see: Mineral Galleries, http://mineral.galleries.com/minerals/silicate/charoite/charoite.htm


    Photo date 2/02, © by S.W. Aber.
    Chiastolite is the opaque variety of andalusite and made of aluminum silicate. It is considerably softer than andalusite, with a hardness of 5 to 5½ (andalusite's hardness is 7½). Chiastrolite ranges in white, gray and yellowish opaques. Chiastolite is valued in its rough form as an amulet. This gemstone is referred to as a cross stone because of its distinct crossing of dark carbonaceous inclusions when viewed perpendicular to the prism's axis. This stone is found in various deposits on every continent but Antarctica, which makes it as valuable as the collector appreciation for it. Notable deposits are found in Algeria, South Australia, Bolivia, Chile, France, Russia, Spain, and in the US, California. (P. Harley)

    Photo date 2/02, © by S.W. Aber.
    Chrysocolla is a gemstone that is not commonly known, but is becoming more popular. It is green or blue in color and has a greasy vitreous luster. It streaks green to white and has hardness of 2 - 4. No cleavage is present and it has a conchoidal fracture. It is a hydrous copper sulfate with its transparency going from opaque to translucent. Fluorescence and pleochroism are both absent. Its' crystal system is monoclinic and it has a density of 2.00 - 2.4. Deposits are found in Chile, Israel, Mexico, Peru, Russia, Zaire, and Nevada. The rough specimen shown is from the Ray Mine, Pinal County, Arizona USA. (S. Kelley)

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


    Photo date 2/02, © by S.W. Aber.
    Chrysoprase is the most valuable type of chalcedony. It is translucent to opaque green with a hardness of 6.5-7. There is no cleavage and the fracture is rough. The color is caused by nickel and can fade under sunlight or heating. It is found in cracks in serpentine rocks and weathered nickel deposits. In the U.S., it is found in California. It is now used in necklaces en cabochon, but was once used as an interior building stone. (L. Flax)

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


    Photo date 2/03, © by S.W. Aber.
    Citrine is a transparent quartz mineral that gets its name from its yellow coloring. Citrine varies in color from light to dark yellow and also golden brown. It has a hardness of 7 and has no cleavage. Citrine has a hexagonal crystal system, with a shape that consists of six triangles forming a pyramid attached to a hexagonal prism. The chemical composition is silicon dioxide, SiO2. Although natural citrine is rare, citrine can be created by heat treating amethysts and smoky quartz. Heat treated citrines tend to have a reddish tint, whereas the natural citrines are usually a pale yellow. Citrine is found in Brazil, Madagascar, the United States, Argentina, Burma, Namibia, Russia, Scotland, and Spain. (K. Gaines)

    Photo date 4/02, © by S.W. Aber.
    Coral is of organic origin and can be used as a gem material. It is used for beads, cabochons, ornamental objects, and sculptures. Coral is a structure produced by "colonies" of animals individually called "polyps". They extract minerals from sea water fo form these structures. The most common mineral in most corals is lime. The coral is found at depths of 1-1020 feet and mainly harvested with weighted nets dredged across the seabed. Three quarters of all coral harvested is processed at the main trade center Torre del Greco, south of Naples, Italy. Coral comes in red, pink, white, blue, and black. It streaks white and has a hardness of 3-4. It has no cleavage and its' fracture is irregular, brittle, and splintery. Unpolished coral is dull, when polished it has a vitreous luster. Corals are sensitive to heat, acids, and hot solutions. It can fade, and some corals are dyed. Black coral consists of an organic horn substance(keratin like). It is shiny and somewhat flexible. It is of no industrial economic importance in world trade. (S. Kelley)
    Return to the top.
    Return to the top.


    Return Gem 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 11, 2012.

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