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/gembrief.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 4/02, © by S.W. Aber.

Visit Postage Stamps featuring
Agate
Petrified Wood
Chalcedony
Carnelian
Chrysoprase
Jasper
by Richard Busch!

Agate is a microcrystalline quartz. Agates are usually found in nodules or filling in a void or vesicle in a volcanic rock. It can be translucent - opaque, in many colors, and is often banded or spotted. It is unknown how the banding forms, but it may be from gradual crystallization from solution inside the matrix rock, or perhaps from silicic acids in gas bubbles as the matrix rock is cooling. Sometimes the center of the agate is not complete, allowing for the formation of crystals. The refractive index is 1.530-1.540, while the specific gravity is 2.60-2.64. Agate has a hardness of 6.5-7 and fluorescence is varible.
Agate comes in many varieties and are sometimes dyed. We have already studied
moss agate and fire agate. Another notable type is scenic agate, where dendrites form to resemble a landscape. Agates can be cut for jewelry or ornamental purposes.
Fossils are also used as gemstones. A fossil is "the remains or traces of ancient life as preserved in rocks" (Morales 2001). The remains/traces are usually preserved by a variety of mechanisms, including a siliceous replacement similar to agates or chalcedony. Examples of fossils include ammonite, ammolite (shown earlier in the course, a piece of ammonite shell) and coprolite (fossilized feces). Fossils can be cut and polished for jewelry and other ornamental purposes such as book ends.
Petrified Wood is fossilized tree parts and used for jewelry and ornamentation. Petrification is a combination of the replacement of cell walls and the permineralization (filling in) of the hollow spaces. Chief material of petrification: silicon dioxides, such as agate and jasper. (L. Flax)
For additional information on agate visit:
  • http://stoneplus.cst.cmich.edu/agate.htm, R. V. Dietrich (2010). GEMROCKS: Ornamental & Curio Stones
  • Casey, Ken (Feb. 2005). Mineral of the month--February: Chalcedony, microcrystalline quartz (variety "agate"). Wilmington, DE: Delaware Mineralogical Society. http://www.delminsociety.net/motm/motm_feb2005.htm
  • OUr Oregon Coast (2012). Agate hunting on the Oregon coast. Our Oregon Coast. http://ouroregoncoast.com/home/beachcombing-and-agates.html
  • Agate. Treasure Hunting Wiki, http://cash-and-treasures-wiki.travelchannel.com/page/Origin+of+Agates#fbid=zNtL4DIc7d_ and Origin of Agates, Agates of the World.
  • South Dakota State Gemstone, http://www.netstate.com/states/symb/gemstones/sd_fairburn_agate.htm, Fairburn Agate.
  • Beautiful Agates (2002), http://agatenodule.com/index.htm

  • Photo date 4/02, © by S.W. Aber.
    Alabaster is the fine-grained variety of gypsum with the IUPAC chemical name of dihydrous calcium sulfate (CaSO4 2H2O). It is generally white in color but can also be pink to brownish. It has a vitreous luster and leaves a white streak. Alabaster is a very soft mineral with a hardness of 2. It falls in the monoclinic crystal system structure, with perfect cleavage in one direction and distinct cleavage in two others. Crystals are usually found as untwined single crystals that display a flat, rhombic form and may appear fibrous along fractured surfaces. It has a conchoidal fracture and appears translucent or pearly on cleavage surfaces. Due to its granular make-up, alabaster is the denser form of gypsum. However, it is still quite light, having a specific gravity of 2.3 to 2.33, due to its porous nature.
    Known deposits of alabaster are found in Thuringia, Germany; Derbyshire, England; Parisian Basin, France; Tuscany, Italy; and in the U.S. in Colorado (Schumann, 1997, p. 222). It develops principally in sedimentary rocks of chemical origin. Due to its low hardness and resiliency, alabaster is rarely used in jewelry, but rather, it is used in objects of ornamentation. It is often dyed different colors because of its porosity. The name alabaster derives from the Greeks, as does the name of gypsum. In antiquity, the name alabaster also referred to microcrystalline limestone due to similar appearance. All varieties of gypsum are ground and used in plasters and cements. The translucent variety of gypsum, known as selenite, may have a silky luster which commercially, can be known as moonstone. (J. Archuleta)
    Schumann, W. (1997). Gemstones of the World. NY: Sterling Publishing Company.
    For additional information on alabaster visit
    http://stoneplus.cst.cmich.edu/alabaster.htm, R. V. Dietrich (2002). GEMROCKS: Ornamental & Curio Stones.


    Image taken from
    www.awesomegems.com
    Alexandrite was named for Czar Alexander II and discovered around 1830 in the Ural Mountains of Russia (Schumann, 1997, p. 98). It is a variety of the mineral chrysoberyl and has a peculiar phenomenon that makes it valued as a gemstone. This mineral is green in daylight and red in incandescent light! The thicker the stone, the more the color change is visible. This stone is sensitive to knocks and color changes are possible with exposure to extreme heat. Thus, it follows that extreme care must be taken when fashioning and wearing this stone. Alexandrite can be found in Burma, Madagascar, Tanzania, Brazil, and Russia. It is currently mined in Sri Lanka and Zimbabwe. According to Schumann (1997), the largest cut alexandrite is 66 ct. and is in the Smithsonian Institution in Washington D.C. (p. 98). (J. Willard)
    Reference:
    Schumann, W. (1997). Gemstones of the world. NY: Sterling Publishing Co., Inc.
    For more information visit the gem course webpage, http://academic.emporia.edu/abersusa/go340/alexandr.htm, and student created pages, http://academic.emporia.edu/abersusa/go340/students/howard/index.htm, http://academic.emporia.edu/abersusa/go340/students/bray/index.htm.

    Photo date 4/02, © by S.W. Aber.
    Visit Postage Stamps featuring
    Amazonite by Richard Busch! amazonite is a (mostly) potassium feldspar, not sodium. The white streaks in the sample you picture are albite, which is a sodium feldspar.
    Amazonite is the bluish green variety of microcline, a potassium feldspar. According to mindat.org, the finest specimens come from the Pikes Peak area in Colorado (USA) but it is also called Amazone stone as it was... "named in 1847 by Johann Friedrich August Breithaupt for an unspecified type locality area "near" the the Amazon River" (/www.mindat.org/min-184.html). Amazonite is an opaque potassium feldspar with white streaks of sodium plagioclase feldspar, likely albite. It is also called Amazone stone and the name was derived from the Amazon. The color distribution is irregular but generally green or bluish green. The luster is vitreous. Deposits can be found in Colorado, Brazil, India, Kenya, Madagascar, Namibia, and Russia. Amazonite has been confused with chrysoprase, serpentine, and turquoise. It has a Moh's hardness of 6-6 and streaks white. The fracture is uneven to splintery and it is brittle. This gem has double refraction, with a refractive index of 1.522-1.530. There is no pleochroism and a weak fluorescence. (P. Mura, edited by S. Aber)

    Photo date 2/02,
    © by Berg and Dougan.
    Visit the amber stamp by Richard Busch!
    Amber is fossilized hardened tree resin. Although it occurs in many colors, white, yellow, orange, red, green, blue, brown, and black, the "amber" orange color is most common. It is brittle, with a conchoidal fracture and hardness of 2-2.5. It is transparent to opaque and most amber is mined in the Baltic Russian Federation, Kalliningrad. (S.W. Aber) To know more about amber visit:
  • http://academic.emporia.edu/abersusa/amber.
  • http://stoneplus.cst.cmich.edu/amber.htm, R. V. Dietrich (2002). GEMROCKS: Ornamental & Curio Stones.

  • Photo date 2/02, © by Berg and Dougan.
    Ammolite is a rare gemstone that is only found in one region in Alberta, Canada. This gem is formed from the compression of the shell of a fossilized animal, the ammonite, which was similar to modern day nautilus. This compression causes many thin layers of aragonite. Because of its limited supply, ammolite is priced between 35 an 60 dollars per carat in its rough state. Ammolite is very soft for a gemstone, with a hardness of about 3.5 on the Mohs scale, so it can only be used if it is impregnated with plastic resin or set as a doublet or triplet. The colors vary from deep red to green, gold, blue and purple. The colors such as red are more durable because the red is made of more layers of aragonite, while purple is thinnest and therefore the least durable. (A. Dougan)

    Photo date 1/03, © by S.W. Aber.
    Amethyst is a stone that is highly valued in that it differs from common quartz because of its violet to purple color. The coloration is caused by the presence of iron and/or manganese. Amethyst has an absorption spectrum of 550-520 and its color can be restored or enhanced by the high energy of X-ray radiation. As a member of the quartz group, it has a hardness of 7, density of 2.65 and streaks white. It occurs in the trigonal crystal system and is composed of silicon dioxide. Amethyst are found in geodes of alluvial deposits. The most significant deposits are in Brazil, Madagascar, Zambia, Uruguay and many others including the United States (Arizona). It is a birthstone for February and is commonly used in jewelry. Ametrine or tristine is half amethyst (purple) and half citrine (yellow) and its deposits are located in Brazil and Bolivia. (P. Harley)

    Photo date 3/02, © by S.W. Aber.
    Andalusite is a multi-colored stone with good cleavage and uneven or brittle fracture. It has a hardness of 7 . It exhibits weak fluorescence (green to yellow-green) and has a specific gravity of 3.05-3.20. Andalusite has exceptional pleochroism and therefore describing the color can be a challenge. It may appear as a dull yellowish green with pink or brown reflections, or a yellowish brown with green or violet reflections. It can be green or a peach color but usually with other colors visible through different facets. Gem quality andalusite is named after Andalusia, Spain, and is rare. The luster is vitreous. Deposits are found in Australia, Brazil, Canada, Russia, Spain, as well as in the United States. It may be confused with chrysoberyl, smoky quartz, sinhalite, tourmaline, or idocrase. Andalusite has a opaque variety called chiastolite. (P. Mura)

    Photo date 3/02, © by S.W. Aber.
    Anyolite is a name given to a massive green zoisite mineral with ruby and hornblende inclusions. It is found in Tanzania. The hardness is 6 and specific gravity, 3.35. It's chemical formula is Ca2Al3(SiO4)3(OH). It has a white streak. The appearance of anyolite is unmistakable and it has not been imitated or synthetically produced yet. It is prized as an ornamental material, usually sculpted and polished into boxes, ashtrays, and small carvings. (A. Dougan)

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

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    Photo date 4/02, © by S.W. Aber.
    Apatite can occur in many colors, but usually green or brown. The gem variety is usually blue. It has a hardness of only 5, which is low for a gem. It has a white streak and a specific gravity of 3.1 to 3.2. Although apatite is the most common of all the phosphorous-bearing minerals, it is not common as a gemstone. It is found in igneous rocks as an accesory mineral, but it can also occur in certain metemorphic rocks as well. Its name comes from the Greek word apate, meaning "deceit" because it is easily confused with other minerals, such as aquamarine, olivine, and fluorite. (A. Dougan)

    Photo date 1/03, © by S.W. Aber.
    Aquamarine is one variety of the mineral beryl. It is found ranging in color from light blue to dark blue and also blue-green. It is most commonly known as the birthstone for March. The word aquamarine is Latin for sea water. Aquamarine has a hexagonal crystal system and is usually transparent, but can be opaque. It has a hardness of 7.5-8 and a refractive index of 1.564-1.596. It is a brittle stone and is sensitive to pressure. The most desired color is dark blue. However, lower quality gems can be heated to 725-800 degrees F to change them to the darker color. This change is permanent. Unfortunately, higher heat will cause discoloration in the stone. Inclusions are rarely found in aquamarine and usually cause the gem to be more fragile. Occasionally a gem cam be found with a cat's eye effect. It is often found in pegmatites and coarse-grained granite. Aquamarine is mined in many places including Brazil, Russia, Australia, Tanzania, Kenya, Nigeria, and many other countries. The largest finding of gemstone quality was in Brazil in 1910. It was 18 inches long and 15 1/2 inches in diameter and weighed in at 243 pounds. It produced many stones with a total weight of more than 100,000 carats. (A. Hess)

    Image taken from
    http://www.theimage.com/
    mineral/aragonite/2.htm


    Photo date 2/03, © by S.W. Aber.

    Aragonite's chemical name is calcium carbonate (CaCO3) and it may contain some strontium, lead, and zinc compounds within its structure. It varies in color and can be white, gray, colorless, yellow, pale green, violet, or brown. Aragonite has a vitreous luster and leaves a white streak. All of the afore mentioned characteristics, including chemical make-up are identical to the more common mineral calcite. However, there are several differences between the two despite their chemical similarity. Aragonite's luster is more resinous while calcite is dull. Aragonite is also a harder, denser mineral with a hardness of 3.5 4 and a specific gravity of 2.9-3.0 (Schumann, 1997, p. 208). Cleavage is also a distinct difference between the two. Calcite has perfect three directional cleavage whereas aragonite has good cleavage in one direction and poor cleavage in two others with a subconchoidal fracture. It is these differences in cleavage that provide the best method of identification between the two. Aragonite crystals are orthorhombic with untwinned crystals being very rare. The twinned crystals show pseudohexagonal prisms and spear- or chisel- shaped forms.
    Aragonite is formed in evaporite deposits of sedimentary rocks associated with gypsum and calcite (stalactites). It is also found in blue schists of regional metamorphic rocks and in massive and disseminated hydrothermal replacement deposits. Locations for occurrence are found in Spain, Mexico, Morocco, New Mexico, and Arizona. It was first discovered in Aragon province, Spain where its name was derived.
    Aragonite is also the primary mineral that makes up the organic compound commonly known as mother of pearl. Mother of pearl is created in a number of mollusks but commercial sources primarily come from oyster-like species. Aragonite is chemically mixed and bonded with water and an organic horn substance, known as conchiolin, that binds the microcrystals together to form the inner, nacreous layer of the mollusk's shell (also bound concentrically around an irritant to form a pearl!) (Schumann, 1997, p. 230). The new material formed may be harder or softer than the isolated mineral (2.5-4.5) depending on the ratio of the mixed chemicals. Mother of pearl may vary in color as well (white, pink, silver, cream, golden, green, blue, or black) due to the type of mollusk and the water. The one constancy lies in the luster and iridescent play of colors, caused by the overlapping platelets of aragonite and film of conchiolin (Schumann, 1997, p. 239). (J. Archuleta)
    Reference: Schumann, W. (1997). Gemstones of the World. NY: Sterling Publishing Company.

    Photo date 2/02, © by S.W. Aber.
    Aventurine quartz is a green variety of quartz. It obtains its green color from inclusions of fuchsite, a green mica. It can also be red to gold-brown due to inclusions of hematite. Aventurine has a chemical composition of SiO2, it is translucent to opaque, and it has a hardness of 7, a white streak, and a specific gravity of 2.64-2.69. (A. Dougan)
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    Return Gem Syllabus or 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: April 5, 2016.

    Copyright 1999-2016 Susan Ward Aber. All rights reserved.