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.

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.

Schumann's (1997) third commercial class was gemstones for collectors, found on pages 204-217. 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. 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.

The final commercial class is organic gemstones, described in the textbook on pages 224-239. 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.

The first two commercial categories listed above 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

• Adamantine is the highest nonmetallic luster category, represented by the brilliant luster of diamond.
• Subadamantine is next lower on the scale and found in high refractive index gems such as zircon and garnet.
• Vitreous, or glassy, is the most common luster in minerals, with quartz and tourmaline as gem examples.
• Subvitreous is next lower on the scale.
• Greasy has an oily appearance and texture, a typical luster of jade. You can feel this luster somewhat.
• Waxy is a descriptive term which can resemble candle wax and is common to chalcedony and turquoise. Waxy luster can have a feel to it as well.
• Dull or earthy is a term used with metallic or nonmetallic material. It is found in most gems before they are polished and may have a tactile property.
• Resinous refers to the shiny luster of resin, with amber as an obvious example.
• Silky is a luster common to satin spar gypsum and tiger's-eye. It is caused by the reflection of light from a parallel, fibrous texture.
• Pearly is a luster that produces a kind of shimmering glow and is found in pearl, mother-of-pearl, and moonstone.

Light Transmission

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 determine 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.

Chromophores

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).

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:

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).

Color Centers

Color Caused by Inclusions

Variation in Color

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

Optical Phenomena

• Adularescence

  Adularescence is the milky or bluish sheen or opalescence, which is best seen in gems cut in cabochon. Moonstone's adularescence is formed by thin lamellae resulting from exsolution (Hurlbut and Kammerling, 1991, p. 74).

• Interference and Diffraction Colors

  • Iridescence is an interference of light reflected from the interior or surface of a gem that can produce a spectrum of colors at different angles. This effect is similar to the look of an oil-water mixture or soap bubbles. The interference colors can be from light diffracted from thin films of liquid or gas, closely spaced fractures, twinning, or exsolution lamellae, and cleavage planes. Iridescence in fire agate is caused by thin films of iron oxide and flame obsidian's iridescence is from internal fractures.
  • Play-of-Colors is displayed when closely packed, uniform, silica spheres have water and air trapped between the spheres. When light passes through the colorless opal and into the water/air which fill the voids, some wavelengths are diffracted out of the stone in pure spectral color bands, called a play-of-colors. Schumann (1997) referred to this phenomenon as opalization (p. 46).
  • Labradorescence or schiller is found in the feldspars, such as labradorite, spectrolite (trade name for labradorite from Finland), larvikite, and moonstone. Blue and green effects are most common, but the entire spectrum can be seen at times.

• Color Change

  Under different lighting conditions, such as incandescent to natural light, some gems will change colors. This is termed the alexandrite effect because it was first observed in the chromium colored variety of chrysoberyl called alexandrite, which can change from red to green. Other minerals, corundum, spinel, tourmaline, and garnet, can exhibit this unusual optical phenomenon. Synthetic alexandrite, synthetic corundum, and lab created glass can also produce distinct color changes.

• Chatoyancy

  Chatoyancy, or descriptively called the cat's eye effect, is a silky sheen produced by the reflection of light by parallel fibers or needlelike inclusions or cavities. When the mineral is cut en cabochon, perpendicular to the fibrous direction, it displays a band of light which resembles the slit eye of a cat. This effect is seen in satin spar gypsum, tiger's eye, tourmaline, beryl, diopside, and chrysoberyl. When the "cat's eye" term is used alone, it refers to chrysoberyl; all other gems exhibiting this phenomenon must have the mineral designation associated with the term (Schumann, 1997, p. 45).

• Asterism

  When needlelike inclusions are parallel to each other, but in a hexagonal pattern or a triple chatoyancy, it is termed asterism or a star effect. Four- and twelve-rayed stars can exist, but most are six-rayed. When brightly illuminated, the cabochon cut (perpendicular to the long axis direction) hexagonal minerals, such as corundum or quartz, can produce rays 60 degrees apart. Star rubies and sapphires produce asterism because of exsolution and acicular rutile.

• Aventurescence

  Aventurescence is a phenomenon produced by small particles included in minerals or glass. Schumann (1997) called this aventurization (p. 45). Aventurine glass, known as goldstone, has a glittering effect that is the result of tiny copper inclusions or a precipitation of copper crystals out of the molten glass (Hurlbut and Kammerling, 1991, p. 78). Natural aventurescence is found in aventurine quartz (fuchsite mica or hematite plates), oligoclase feldspar called sunstone (hematite plates), and Oregon sunstone (copper plates).

Required reading! For further explanation see: