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/gb3.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.

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Photo date 2/02,
© by Berg and Dougan.
Odontolite, also called toothstone, is a fossilized tooth or bone of an extinct prehistoric animal, such as mammoth, mastodon, or dinosaur. It has become a popular substitute for ivory. Odontolite may also be blue if the fossil bone is colored turquoise-blue by the mineral vivianite (iron phosphate). (S.W. Aber)
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Photo date 2/02,
© by Berg and Dougan.
Obsidian is a black, brown, or black and red colored, compact natural glass. It is therefore amorphous and a mineraloid with the hardness of glass, about 5.5. Nodular obsidian fragments that are transparent are termed Apache Tears. They can be naturally rounded due to water erosion and abrasion. Snowflake Obsidian has inclusions of the mineral cristobalite. The crystals have grown when this volcanic rock was still a hot, viscous melt that subsequently cooled and solidified. (S.W. Aber)

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


Photo date 2/02,
© by Berg and Dougan.
Onyx Marble is a translucent to opaque, yellow-green, white, or brown striped rock. Marble is metamorphic limestone or travertine, and onyx marble is formed from layered deposits in warm springs and caves. Hardness is 3.5-4 similar to most marbles. It is often dyed and called Mexican onyx. See pg. 219 in textbook for pictures. (L. Flax)

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


Photo date 3/02,
© by S.W. Aber.
Onyx or black agate is a stone that is formed in layers. The layers usually consist of a combination of color, alternating black and white layers. The name onyx is also used to describe unicolored chalcedony like black onyx. When onyx is cut into cabochons or beads the black and white layers often alternate. Sand onyx is a stone with a brown base and white upper layer. Carnelian onyx or sardonyx is a stone with a red base and white upper layer. Onyx is often times confused with onyx marble, which is not onyx at all. The word onyx comes from the Greek word meaning fingernail most likely from its low transparency. (J. Sielert)

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


Photo date 2/03, © by S.W. Aber.
Peridot is a variety of the mineral olivine, and is the birthstone for August. It ranges in color form yellowish green to dark lime green. There are also brown varieties, although they are less desirable. The chemical make-up of peridot is (Mg, Fe)2SiO4. The color is determined by the ratio of Mg to Fe. Peridot has an orthorhombic crystal. It is fairly had with a hardness of 6.5 7. It is a fairly dense gem, with a specific gravity of 3.27 3.37. Peridot has a brittle, conchoidal fracture and an imperfect cleavage.

Peridot is usually found in association with basalt and gabbro in volcanic ejecti. It is found in many locals, however the largest amounts comes from St. John's Island in the Red Sea. It is found in gem quality in the United States on the San Carlos Apache Reservation in Arizona, which is currently the largest known deposit of gem peridot. It is also found in New Mexico, although these are typically small and not good quality stones.

Peridot has three refractive indexes, two fixed and one variable. The numerical difference between the highest and lowest refractive indexes is fairly large, causing a double image to be viewed through the gem. This causes the facet junctions to be doubled, but also the inclusions. Substitutes for peridot include synthetic sapphire, synthetic spinel, and glass, although these will show no doubling. The largest known cut peridot is housed in The Smithsonian, and is 310 carats. (K. Gaines)

References:

  • http://jewelry.about.com/library/weekly/aa082602a.htm
  • http://www.theimage.com/gemstone/peridot/peridot.html

  • Photo date 2/03,
    © by S.W. Aber.
    Petrified wood is fossil wood used for jewelry and ornamentation. One fossilization process includes converting organic matter into stone through the impregnation of dissolved minerals. This process is known as petrifaction. The dissolved silicon dioxide that reform as "petrified wood" include jasper, the chalcedony species and occasionally opal. Specimens range in colors such as brown, gray, red and yellow. Petrified wood leaves a slightly colored white streak and ranges in hardness from 6.5 - 7 based on its composition. Petrified wood is a microcrystalline aggregate with no cleavage and an uneven, splintery fracture. The most significant petrified forest is located in Arizona featuring specimens up to 213 feet long and 10 feet thick (Schumann, 1997, p. 148). (P. Harley)

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


    Photo date 2/03,
    © by S.W. Aber.


    Photo date 2/03,
    © by S.W. Aber.


    Image taken from:
    www.rocksandminerals.com/
    specimens/pipestone.htm
    .

    Pipestone, also known as catlinite, is a form of claystone called argillite by petrologists. Argillites are claystones or shales that have undergone metamorphic processes but they lack the distinct lamination and cleavage of slates. Pipestone is made up of fine-grained sericite with smaller amounts of hematite, pyrite, and possible rutile. It is generally a deep red to pale orange in color and has a hardness of 2.5. It formed approximately 1.6 to 1.8 billion years ago between layers of quartzite. Pipestone was discovered in southwestern Minnesota by the Sioux Indians and was used to carve pipes and other ceremonial objects. George Catlin, a known American artist and writer, visited the Sioux in the area in 1936. He collected samples of the stone and found it has a unique chemical composition. It is believed that this type of stone is only found in this area and was named catlinite in his honor. It is extremely easy to carve because of its lack of quartz. Pipestone National Monument, near Pipestone, MN, was established in 1937 at the site in Minnesota. It is a 282 acre area and is now quarried only by American Indians. There is a general confusion on whether or not Pipestone National Monument is the only site containing catlinite. Other sites given are Rice Lake, Wisconsin; Garretson, Wisconsin; Norton, Kansas; and South Dakota. Many of these sites produce an argillite pipestone, but of a different composition than that in Minnesota. Some of the stone from these areas are often misrepresented as catlinite, but do not have the special carving abilities of catlinite. The pipestone found at the Kansas site is found in glacial drift along with Sioux quartzite boulders. (A. Hess)

    References:


    Photo date 2/02,
    © by S.W. Aber.

    Visit the rhodochrosite stamp
    page by Richard Busch!

    Rhodochrosite is a red to pink mineral that is tranparent to opaque with a vitreous luster. It has a white streak, a relatively low hardness of 4, and specific gravity of 3.45 to 3.70. Rhodochrosite is Greek for rose colored and it is also called manganesespar and raspberryspar. It has only been on the market since 1940. The most important deposits of rhodochrosite are in Argetina, but it is also found in Chile, Mexico, Peru, South Africa, and the U.S. (J. Berg)

    For additional information on rhodochrosite visit:

  • http://stoneplus.cst.cmich.edu/rhodochrosite.htm, R. V. Dietrich (2002)
  • GEMROCKS: Ornamental & Curio Stones

  • Photo date 2/02,
    © by S.W. Aber.

    Visit the rhodonite stamp
    page by Richard Busch!

    In addition to rhodonite's red color, this gem usually has black dendritic inclusions of pyrolusite, a manganese oxide. This gem is opaque, that is transparent varieties are very rare. It has a vitreous luster; on cleavage faces, there is a pearly luster. Streak is white, hardness of 5.5 to 6.5. It is part of the triclinic crystal system. It is cut with a table or en cabochon for necklaces and ornamental objects. (K. Barnett)

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


    Photo date 4/02,
    © by S.W. Aber.

    Visit the rutilated quartz stamp
    page by Richard Busch!

    Rutilated quartz is usually a crystal or smoky quartz with inclusions of rutile. The rutile a is golden looking needlelike mineral which can form a star-like pattern or can look like golden hair in the quartz. Being quartz, it has a hardness of 7, a white streak, and a conchoidal fracture. Rutilated quartz is one of few gems with inclusions that is desired and actually worth more than the pure quartz without the inclusion!
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    Return to GO 340 Syllabus or go on to 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.