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

http://academic.emporia.edu/abersusa/go340/visual.htm



Image left: Crystal ball of quartz, 242,323 carats.
Sphere cut/polished in China, 1923-24. Photo by S.W. Aber,
4/2009, taken at Smithsonian National Museum of Natural History

Visual Properties

In addition to the classification of gem minerals by crystalline structure (crystal systems) and chemical composition (mineral or chemical classes), Schumann (1997) provided a commercial classification which determined the arrangement of gemstones in his book (p. 69). The categories are best known, lesser known, gemstones for collectors, rocks, and organic gemstones, and are listed below.

    The best known gemstones include diamond, corundum (ruby/sapphire), beryl (emerald and other varieties), chrysoberyl (alexandrite/cat's eye), spinel, topaz, garnet, zircon, tourmaline, spodumene (kunzite/hiddenite), quartz (many varieties), opal (many varieties), jade (jadeite and nephrite), peridot, zoisite (tanzanite and other varieties), hematite, pyrite (marcasite), feldspar (moonstone, sunstone, and other varieties), rhodochrosite, rhodonite, turquoise, lapis lazuli, sodalite, azurite, and malachite. Most of these gems have high hardness and/or a tough tenacity and are detailed in your textbook, Schumann (1997), on pages 70-177 in my edition.
Best known multicolored sapphires from Montana (left), and Columbian emerald (right); lesser known diopside (center) is from Finland.
The emerald are set in an art deco necklace with graduated emerald drops and small emerald beads all set in platinum and diamonds.
The necklace was made by Cartier in 1928-29. Photos by S.W. Aber, 4/2009, taken at Smithsonian National Museum of Natural History.
    Lesser known gemstones are usually not found in jewelry stores but rather at gem and mineral shows or rock shops. These gemstones include andalusite, cordierite (iolite or "water sapphire"), diopside (green color, black star diopside, or black cat's eye), apatite (blue or other colors), sphene (titanite), fluorite (often bi-colored), chrysocolla, serpentine, and tiger's eye matrix, to name a few, and are described in the textbook, on pages 178-203 in my edition.
Image left, turquoise. Image center, ruby. Image right, malachite. While all may
be better known, the ruby form shown would not likely be used for jewelry.
Photos by S.W. Aber, 4/2009, taken at Smithsonian National Museum of Natural History.
    Schumann's (1997) third commercial class was gemstones for collectors, found on pages 204-217 in my edition. Although some are common rock-forming minerals, these gems are cut by a lapidary for ornamental display and are usually not set in jewelry. Some examples include tugtupite, calcite, sulfur, gypsum, and muscovite/lepidolite.
    The fourth class, rocks as gemstones, are shown in the textbook (Schumann, 1997) on pages 218-223 in my edition. Although Schumann considers these on the "fringe zone of gems," many are commonly seen, such as: onyx marble (Mexican onyx), which is banded and commonly dyed for use in sculpted pieces such as chess sets and fruit; landscape marble, which with a little imagination provides a city view or landscape in the fine grained limestone; obsidian, also known as apache tears and snowflake obsidian; alabaster, a fine-grained gypsum, sculpted into figurines and lamps; meerschaum, which is used in pipes and cigarette holders.
Image left, a zinc carbonate called smithsonite, which is named after James Smithson, founder of the Smithsonian and a mineralogist.
Image right, bloodstone-quartz from India and chrysoprase, a variety of quartz from Marlborough mine, Queeensland, Australia.
These minerals may not be well-known to all but smithsonite as a mineral specimenis highly sought after and chrysoprase,
quite valued in Poland and Australia. Photo by S.W. Aber, 4/2009, taken at Smithsonian National Museum of Natural History.
    The final commercial class is organic gemstones, described in the textbook on pages 224-239 in my edition. These gems include amber, pearl, coral, ivory, jet, and mother-of-pearl, from bivalves such as the Paua mussel of New Zealand. Some lapidarists work with Spondylacea oysters, and specifically the living variety Spondylus, also known as thorny oysters (fossil specimens lack coloration). These organisms have shells with brilliant hues of orange, yellow, crimson, and violet and are found in the Pacific.


Golden and silver pearls from Jewelmer; photo by S.W. Aber 2011, Tucson Gem Shows. Amber workshop in Palanga Amber Museum and Wikipedi; photo by J.S. Aber 2001.


The first two commercial categories listed above, best known and lesser known, are mostly transparent to translucent and identified using optical properties and testing equipment such as the polariscope and refractometer. The final three categories include many opaque minerals, rocks, and organic substances. Identification of these specimens can be problematic. Optical property testing may not be possible and chemical or physical property testing can be destructive to a fashioned gem.

In the past, gemologists dealt mostly with a few natural gems, unsophisticated synthetics, or assembled stones, glass, and plastic. Many more convincing synthetics and enhanced gems exist today, as well as a greater variety of material considered as gems. Also, some traditional stones being mined in new localities have resulted in a wider range of properties. The gem and created stone diversity that exists today has led to a complexity in identification, which has caused a proliferation of gem testing instruments; but, the trained human eye is the most important instrument for the gemologist or connoisseur of gems and jewelry (Hurlbut and Kammerling, 1991, p. 67). Careful observation using visual and tactile properties are the useful clues to identification of many gemstones. This lecture includes an introduction to some of these visual properties.

Luster

Luster refers to the quantity and quality of light that returns to the eye under normal lighting conditions. In other words, what does it look like?! Minerals are divided into two general luster categories, metallic and nonmetallic. Metallic luster is used for opaque material with a look of metal, either dull like a nail or bright and shiny like a polished gold ring. Pyrite, marcasite, and hematite are examples of minerals with metallic luster and used in jewelry. Marcasite is the mineral name associated with a particular style of jewelry, but in fact this material used as the gem is actually pyrite. Submetallic luster is intermediate and found in minerals such as sphalerite. Nonmetallic luster, used in transparent or translucent material, can be divided into many categories, from adamantine to pearly, with categories ultimately related to the gem's refractive index.

Image left, gold has a metallic luster. Image center, diamonds have an adamantine luster.
Image right, turquoise with a high polish giving it a resinous luster.
Photos by S.W. Aber. Two left, were taken at Smithsonian National
Museum of Natural History, 4/2009; image right was taken at the 2010
Tucson Gem Shows of a crown on a traveling exhibit from the Smithsonian NMNH.

Light Transmission

Many gems transmit light and are called transparent or translucent, depending upon how much light passes through. Gems that do not transmit light, even when viewing a thin slice, are truly opaque, such as hematite and pyrite. The transmission of light is dependent upon the amount of light reflectance and absorption. If an object can be seen distinctly through a gem it is termed transparent. If the object is indistinct, the gem is semitransparent. Light passing through, but not enough to distinguish an object, is termed translucent or semitranslucent.

Color

Nature of Light

Color, or the lack of, is a major factor in the beauty of gem materials. Although color is a constant property, minerals, such as quartz and beryl, can come in a wide range of different colors. There are several explanations for the cause of color and some will be briefly introduced.

Particle or quantum theory and wave theory are both used to explain light and color, visual and optical properties of minerals. Quantum theory regards light as particles or bundles of energy called quanta or photons, whereas wave theory regards light as transmitted energy through electromagnetic waves.

Visible light is a small portion of the electromagnetic spectrum, from 750 to 350 nm, between the infrared and ultraviolet portions of the spectrum. As wavelength decreases from 750 nm, the color varies from red to orange, yellow, green, blue, and violet (at 350 nm). White light is a combination of all the visible wavelengths and appears when no reflected or refracted light is absorbed and all wavelengths are transmitted back to the eye. If some wavelengths are absorbed, termed selective absorption, then the combination of remaining wavelengths that return to the eye determines the color perceived. If a stone is red, then the blue-violet wavelengths of light have been absorbed, while red wavelengths are transmitted. The eye cannot discern subtle differences in hue and therefore two minerals that appear to be of a similar green color, could be the result of absorption of different wavelengths for each mineral.

Garnet and labradorite in many colors. Photos by S.W. Aber, 4/2009,
taken at Smithsonian National Museum of Natural History.

Chromophores

Electrons, negatively charged particles, exist at different energy levels within an atom. The electrons at the highest levels are in the outer orbitals, which can be completely or partially filled with electrons. Radiant energy (photons) of light can enter a crystal and elevate an electron to a higher orbital, if it is a partially filled energy level, and be absorbed in the process (Hurlbut & Kammerling, 1991, p. 70). This selective absorption of wavelengths, and electron oscillation between orbital levels, can cause color and fluorescence.
Image left, ruby-red corundum. Image right, sapphire.
Photos by S.W. Aber, 4/2009, taken at Smithsonian
National Museum of Natural History.

The so-called transition metal elements, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, and copper, all have the outer orbitals partially filled. These elements, when present as a major portion of the chemical formula or as impurities within a crystal, are the common coloring agents of minerals. Therefore, it is the chromophores or the electronic configuration of ions of transition elements present in the crystal's structure that can produce color. One transition element can produce different colors when occupying different sites within the crystal structure.

Different oxidation states of elements can also influence absorption. Ferrous iron (Fe2+) creates green, but ferric iron (Fe3+), found in a similar crystal structural site, can produce the yellow. Oxidation state is the reason heat-induced gem enhancement can intensify a color or create more desirable colors (Hurlbut & Kammerling, 1991, p. 70).

Idiochromatic Minerals

Idiochromatic, or "self-colored" minerals, owe their color to chromophores, or elements, that are essential or major constituents in the chemical formula (Scovil, 1996, p. 60).

Examples of Idiochromatic Minerals
Mineral Color Formula (coloring agent bold-faced)
malachite green Cu2CO3(OH)2
dioptase green Cu6(Si6O18) 6H2O
Azurite blue Cu3 (CO3) 2(OH)2
Cuprite red Cu2O
Sulfur yellow S
Rhodochrosite pink MnCO3
Rhodonite pink MnSiO3
Vanadinite orangy-red Pb5(VO4) 3Cl

Left and center: rhodochrosite, manganese carbonate. Right: Rhodonite
Photos by S.W. Aber, 4/2009, taken at Smithsonian National
Museum of Natural History.

Allochromatic Minerals

Allochromatic minerals, or "other-colored" minerals, are colored by ways other than simply the constituents of the chemical composition. In many gem minerals, the major element in the chemical composition is colorless in a pure state. If these gems occur in a variety of colors, then it is the result of the substitution of one major element for another, impurities, or defects within the crystal structure (Scovil, 1996, p. 60).

An example of major element, or chromophore, substitution is seen in nephrite jade, which is white in a pure state. Green or even black nephrite jade is more common than white and occurs when iron replaces the magnesium in the crystal structure; some substitution creates green and if the substitution is extensive, black. An example of allochromatic coloration with trace amounts of impurities, or chromophores such as iron, chromium, and manganese, is in beryl, 3BeO Al2O3 6SiO2:

Beryl Color Reason
Emerald deep green chromium, Cr3+
Aquamarine light blue Iron, Fe2+>Fe3+
Heliodor yellow Iron, Fe2+>Fe3+
Morganite pink Manganese, Mn2+
Bixbite red Manganese, Mn2+

In allochromatic minerals how can one element be responsible for two different colors? Chromium is responsible for the green color of emerald, the red color of ruby, and the red and green color of alexandrite! Ruby is the red variety of the colorless mineral corundum, Al2O3; the red results when about 1% of the aluminum (Al3+) are replaced with chromium ions (Cr3+). The red color is created by the absorption of light by the chromium in the yellow-green and violet portions of the visible light spectrum, and transmission in the red and minor amounts of blue spectral colors. The absorption and transmission of certain wavelengths of light depends on the electron configuration in chromium, but also on the crystal structure the ions are embedded in. In emerald, 3BeOAl2O36SiO2, the chromium substitutes for the aluminum, but the beryllium and silicon oxides change the crystal structure and thus the frequencies of light that are absorbed. Emerald absorbs violet and yellow-red portions of the spectrum, transmitting the green-blue wavelengths. Alexandrite, or chrysoberyl, is a mineral with a chemical formula between that of corundum and beryl, BeOAl2O3. Chromium replaces aluminum ions, which interact with the beryllium oxide structure, resulting in both red and green colors. Candlelight has abundant yellow and red light, and when passed through the alexandrite the mineral absorbs the blue wavelengths present and transmits the red. Daylight contains more blue wavelengths, so that this mineral appears green-blue and absorbs the red light.

Pseudochromatic

Pseudochromatic, or "false color" minerals owe their color to the physical crystal structure (Scovil, 1996, p. 60). Pseudochromatic mineral examples include opal and labradorite. Opal is amorphous, or lacking in crystal structure, made up of silica spheres roughly arranged in a hexagonal pattern, with 4-20% (or more!) water content. The water and air trapped between the silica spheres act to break up the white light into component colors, allowing for spectral colors, when in fact the "white" opal is colorless (black opal and fire opal have dark body colors). The feldspar, labradorite, is colored by a phenomenon called labradorescence, which is the break up of white light into spectral colors as a result of polysynthetic twinning (alternating microscopically thin layers or lamellae).

Left: Colorless quartz with the reddish color due to rutile inclusions.
Right: Colorless quartz with greenish color due to chlorite inclusions.
All photos above by S.W. Aber, 4/2009, taken at Smithsonian
National Museum of Natural History.

Color Centers

Color centers or F centers cause color in minerals when there is a crystal structure defect or imperfection. The defect can be due to excess or deficient ions of an element in the chemical formula, substitutional impurities, or mechanical deformation within the crystal structure (Hurlbut and Kammerling, 1991, p. 71).

Color Caused by Inclusions

Minerals can be colored by the presence of inclusions, minerals incorporated into minerals. Small particles of copper can produce the orangy sparkle in sunstone, a type of labradorite feldspar, whereas iron oxide inclusions (hematite or goethite) also produce the orangy sparkle in the oligoclase feldspar sunstone. A white mineral called cristobalite can be included in black natural glass and is called snowflake obsidian. Quartz can be colored by impurities, crystal structural defects, or inclusions:

Quartz/Chalcedony Color Reason
amethyst quartz purple iron as (FeO4)4- color centers
smoky quartz or cairngorm brown or black Al3+ > Si4+ plus H+, eject one of a pair of electrons from O2-, (AlO4)4- color center
rose quartz pink Titanium, Ti4+
citrine quartz yellow or orange iron
milky quartz white minute fluid inclusions
greenish-blue chalcedony greenish-blue chrysocolla inclusions
chrysoprase chalcedony yellowish-green nickel
carnelian chalcedony orange hematite or iron hydroxide, goethite
aventurine quartz green fuchsite (chrome bearing muscovite mica) included in colorless quartzite
moss agate chalcedony colorless and dark green chlorite and black manganese oxide inclusions
jasper green or red green or red clay mineral inclusions
fire agate chalcedony brown with iridescence iron oxide inclusions
bloodstone green with orange spots dark green chalcedony with iron oxide or hematite inclusions
prase chalcedony green hornblende or chlorite inclusions
plasma green actinolite inclusions

Carved stones cut from color-banded, micro-crystalline quartz are called cameos.
These combinations above are white chalcedony and black chert or onyx,
as well as red-and-white, sardonyx chalcedony.
Rose, citrine, and smoky quartz.
Brazilian agate - bowl geode with lid, quartz-variety, jasper from Idaho;
agate from Apache Ranch, Chihuahua, Mexico.
Left - blue chalcedony and quartz agate with dendrites of manganese oxide.
Center - ametrine - quartz, natural mixture of citrine and amethyst.
Right - Natural Brazilian rock crystal chalcedony concentric lollipop!
All photos above by S.W. Aber, 4/2009, taken at
Smithsonian National Museum of Natural History.

Variation in Color

Several minerals are bi-colored or vary in color within a single crystal. Watermelon tourmaline can have concentric coloration with green surrounding red or have a zonal arrangement with one color at either end. Quartz is bi-colored with yellow citrine and purple amethyst called ametrine. Topaz and fluorite can also have multi-colored bands within a single crystal.

All photos above by S.W. Aber, 4/2009, taken at
Smithsonian National Museum of Natural History.

Some gems have variation of a single color, such as shades of purple bands in quartz and straight or hexagonal color banding in blue sapphire (synthetic sapphire produced by flame fusion can have curved color banding). Malachite and rhodochrosite are identified by their characteristic color banding; malachite is different shades of green, while rhodochrosite is pink and called the "bacon strip effect" (Hurlbut and Kammerling, 1991, p. 73).

Color Alteration/Enhancements

Color caused by artificial enhancements and heating will be covered under gem creation and enhancement, later in the course.

Optical Phenomena

Color due to special optical phenomena are seen in visible light and can be instructive in identification. These phenomena are discussed below.

Oregon Sunstone. Photo by S.W. Aber, 4/2009, taken at
Smithsonian National Museum of Natural History.

Recommended reading. For further explanation see:

  • http://www.galleries.com/minerals/property/color.htm, What is Important about the Color of Minerals?
  • nature.berkeley.edu/classes/eps2/wisc/Lect7.html Color in minerals. This lecture from Jill Banfield's Gems and Gem Materials course at UC Berkeley.
  • http://www.ganoksin.com/borisat/nenam/optic1.htm, Some notes on optical effects in gemstones, by C. Lewton-Brain of Ganoksin Jewelry Co., Ltd.
  • Causes of Color in Minerals and Gemstones, Paul F. Hlava, Sandia National Lab, Albuquerque, NM.
  • http://www.enmu.edu/services/museums/miles-mineral/colors.shtml, Colors in Minerals, Eastern New Mexico University.
  • http://www.minsocam.org/msa/collectors_corner/arc/color.htm, The Origins of Color in Minerals, by Kurt Nassau (1978). American Mineralogist, vol. 63, p. 219-229.
  • http://minerals.gps.caltech.edu/COLOR_Causes/ Colors of Minerals, the Cause

  • The material for this section came primarily from:

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    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: 1999; last update: August 30, 2012.

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