My watch list
my.chemeurope.com  
Login  

Gemstone




A gemstone, gem or also called precious or semi-precious stone is a highly attractive and valuable piece of mineral, which — when cut and polished — is used in jewelry or other adornments.[1] However certain rocks, (such as lapis-lazuli) and organic materials (such as amber or jet) are strictly speaking not minerals, but are still applied in jewelry and adornments, and are therefore often considered to be gemstones as well. Some minerals that are too soft to be generally applied in jewelry may still be considered gemstones because of their remarkable color, lustre or other physical properties that have aesthetic value. Rarity is another characteristic that lends value to a gemstone.

 

Additional recommended knowledge

Contents

Characteristics and classification

Gemstones are described by gemologists using technical specifications. First, what is it made of, or its chemical composition. Diamonds for example are made of carbon (C) and rubies of aluminium oxide (Al2O3). Next, many gems are crystals which are classified by crystal system such as cubic or trigonal or monoclinic. Another term used is habit, the form the gem is usually found in. For example diamonds, which have a cubic crystal system, are often found as octahedrons.

Gems are classified into different groups, species, and varieties. For example, ruby is the red variety of the species corundum, while any other color of corundum is considered sapphire. Emerald (green), aquamarine (blue), bixbite (red), goshenite (colorless), heliodor (yellow), and morganite (pink) are all varieties of the mineral species beryl.

Gems have refractive index, dispersion, specific gravity, hardness, cleavage, fracture, and lustre. They may exhibit pleochroism or double refraction. They may have luminescence and a distinctive absorption spectrum.

Material or flaws within a stone may be present as inclusions. The gem may occur in certain locations, called the "occurrence."

Value of Gemstones, Past and Present

  The diamond is a gemstone which has its value determined in a different way than most or even all other gemstones. Traditionally the diamond was valued mostly for its physical properties such as hardness and brilliance, not unlike any other (colored) gemstone, and since a diamond was not particularly rare, [2], other (colored) gemstones such as a sapphire or a ruby were valued higher than a diamond for a long time. Although the large diamond finds in South Africa created an even larger supply of diamonds, its trade in the 20th century also became strongly regulated by a single organization: De Beers. [3]. Both their monopoly of the market and their continuous marketing campaign in the last 50-75 years have greatly influenced the perceived value of a diamond, unlike that of any other (colored) gemstone.

The remainder of this article deals with colored gemstones and we refer to the diamonds section for a discussion about diamonds.

A valuable (colored) gemstone is prized especially for its great beauty, rarity or aesthetics. Although color plays a very important role in determining the value of a gemstone, many other factors influence its price as well: market supply (think of the fluctuations of Tanzanite prices), rarity (Red Beryl), popularity of a stone, market mechanisms etc.

General physical characteristics that make a colored stone valuable are color, clarity to a lesser extent (emeralds will always have a number of inclusions), cut, unusual optical phenomena within the stone such as color zoning, and asteria (star effects). The Greeks for example greatly valued asteria in gemstones, which were regarded as a powerful love charm, and Hellen of Troy was known to have worn star-corundum. [4]

A factor in determining the value of a gemstone is called water. Water is an archaic term that refers to the combination of color and transparency in gemstones; used hierarchically: first water (gem of the finest water), second water, third water, byewater. [5]

Historically gemstones were classified into precious stones and semi-precious stones. Because such a definition can change over time, vary per culture and can depend on so many factors, it has always been a difficult matter to determine what constitutes precious stones. [6]

Aside from the diamond, the ruby, sapphire, emerald, pearl (strictly speaking not a gemstone) and opal [6] have also been considered to be precious. At least at the beginning of the 20th century these stones were considered precious more often than not because of their beauty. Up to the discoveries of bulk amethyst in Brazil in the 19th century, amethyst was considered a precious stone as well, going back to ancient Greece. Even in the last century certain stones such as aquamarine, peridot and cat's eye have had their days of increased popularity and hence value [6], much like present day.

Nowadays such a distinction is no longer made by the trade. [5] Many gemstones are used in even the most expensive jewelry, depending on the brand name of the designer, fashion trends, market supply, treatments etc. Nevertheless, diamonds, rubies, sapphires and emeralds still have a reputation that exceeds those of other gemstones.

Rare or unusual gemstones, generally meant to include those gemstones which occur so infrequently in gem quality that they are scarcely known except to connoisseurs, include andalusite, axinite, cassiterite, clinohumite and bixbite.

Gems prices can fluctuate heavily (such as those of Tanzanite over the years) or can be quite stable (such as those of diamonds). In general per carat prices of larger stones are higher than those of smaller stones, but popularity of certain sized stone can jade prices considerably. Typically per carat prices can range from $5/carat for a normal Amethyst to 20.000-50.000 for a collector's 3 carat pidgeon-blood almost "perfect" ruby.

Grading

In the last two decades there has been a proliferation of certification, not only for diamonds but for gemstones as well. There are five [5] major laboratories which grade and provide reports on gemstones.

  • Gemological Institute of America (GIA)
  • American Gemological Society (AGS) is not as widely recognized nor as old as the GIA but garners a high reputation.
  • American Gem Trade Laboratory which is part of the American Gem Trade Association (AGTA) the largest trade organization of jewelers and dealers of colored stones
  • American Gemological Laboratories (AGL) which was recently taken over by "Collector's Universe" a NASDAQ listed company which specializes in certification of many collectables such as coins and stamps
  • European Gemological Laboratory (EGL).

Although certification can provide certainty and clarity, each laboratory has its own methodology to evaluate gemstones; grading experience is different and depending on the cert required each lab approaches these issues differently. Consequently a stone can be called "pink" by one lab while another lab calls it "Padparadscha". One lab can conclude a stone is untreated, while another lab concludes that it is heat treated. [5] Countries of origin has sometimes been difficult to find agreement on due to the constant discovery of new locations. Gem labs need time to study them. Moreover determining a "country of origin" does not have the exact scientific methods at its disposal as other aspects of a gem (such as cut, clarity etc.) [7]

Gem dealers are fully aware of the differences between gem laboratories and will make use of the discrepancies to obtain the best possible cert [5]. One such example is to make use of the differences in "Country of Origin": a sapphire from Kashmir (celebrated for its cornflower blue color) commands four times the price of the same stone from Ceylon and twice the price as a similar stone from Burma. [8]

 

Cutting and polishing

A few gemstones are used as gems in the crystal or other form in which they are found. Most however, are cut and polished for usage as gemstones. The two main classifications are stones cut as smooth, dome shaped stones called cabochons, and stones which are cut with a faceting machine by polishing small flat windows called facets at regular intervals at planned angles.

Stones which are opaque such as opal, turquoise, variscite, etc. are commonly cut as cabochons. These gems are designed to show the stone's color or surface properties as in opal and star sapphires. Grinding wheels and polishing agents are used to grind, shape and polish the smooth dome shape of the stones.[9]

Gems which are transparent are normally faceted, a method which shows the optical properties of the stone’s interior to its best advantage by maximizing reflected light which is perceived by the viewer as sparkle. There are many commonly used shapes for faceted stones. The facets must be cut at the proper angles, which varies depending on the optical properties of the gem. If the angles are too steep or too shallow, the light will pass through and not be reflected back toward the viewer. Special equipment, a faceting machine, is used to hold the stone onto a flat lap for cutting and polishing the flat facets.[10] Rarely, some cutters use special curved laps to cut and polish curved facets.

Gemstone color

Color is the most obvious and attractive feature of gemstones. The color of any material is due to the nature of light itself. Daylight, often called white light, is actually a mixture of different colors of light. When light passes through a material, some of the light may be absorbed, while the rest passes through. The part that isn't absorbed reaches our eye as white light minus the absorbed colors. A ruby appears red because it absorbs all the other colors of white light - blue, yellow, green, etc. - except red.

The same material can exhibit different colors. For example ruby and sapphire have the same chemical composition (both are corundum) but exhibit different colors. Even the same gemstone can occur in many different colors: sapphires show different shades of blue and pink and "fancy sapphires" exhibit a whole range of other colors from yellow to orange-pink, the latter called "Padparadscha sapphire".

This difference in color is based on the atomic structure of the stone. Although the different stones formally have the same chemical composition, they are not exactly the same. Every now and then an atom is replaced by a completely different atom (and this could be as few as one in a million atoms). These so called impurities are sufficient to absorb certain colors and leave the other colors unaffected.

As an example: beryl, which is colorless in its pure mineral form, becomes emerald with chromium impurities. If you add manganese instead of chromium, beryl becomes pink morganite. With iron, it becomes aquamarine.

Several gemstone treatments actually make use of the fact that these impurities can be "manipulated", thus changing the color of the gem.

Treatments applied to gemstones

  Gemstones are often treated to enhance the color or clarity of the stone. Depending on the type and extent of treatment, they can affect the value of the stone. Some treatments are used widely and accepted in practice because the resulting gem is stable, while others are not accepted most commonly because the gem color is unstable and may revert to the original tone.[11]

Heat

Heat can improve gemstone color or clarity. Most Citrine is made by heating amethyst, and partial heating with a strong gradient results in ametrine - a stone partly amethyst and partly citrine. Much Aquamarine is heat treated to remove yellow tones, change the green color into the more desirable blue or enhance its existing blue color to a purer blue. [12] Nearly all Tanzanite is heated at low temperatures to remove brown undertones and give a more desirable blue/purple color. A considerable portion of all sapphire and ruby is treated with a variety of heat treatments to improve both color and clarity.

Radiation

Most blue topaz, both the lighter and the darker blue shades such as "London" blue, has been irradiated to change the color from white to blue. Some improperly handled gems which do not pass through normal legal channels may have a slight residual radiation, though strong requirements on imported stones are in place to ensure public safety. Most greened quartz (Oro Verde) is also irradiated to achieve the yellow-green color.

Waxing/oiling

Emeralds containing natural fissures are sometimes filled with wax or oil to disguise them. This wax or oil is also colored to make the emerald appear of better color as well as clarity. Turquoise is also commonly treated in a similar manner.

Fracture Filling

Fracture filling has been in use with different gemstones such as diamonds, emeralds and sapphires. More recently (in 2006) "Glass Filled Rubies" received a lot of publicity. Rubies over 10 carat (2 g), particularly sold in the Asian market with large fractures were filled with lead glass, thus dramatically improving the appearance of larger rubies in particular. Such treatments are still fairly easy to detect.

Synthetic and artificial gemstones

Some gemstones are manufactured to imitate other gemstones. For example, cubic zirconia is a synthetic diamond simulant composed of zirconium oxide. The imitations copy the look and color of the real stone but possess neither their chemical nor physical characteristics. However, true synthetic gemstones are not necessarily imitation. For example, diamonds, ruby, sapphires and emeralds have been manufactured in labs to possess very nearly identical chemical and physical characteristics to the naturally occurring variety. Synthetic corundums, including ruby and sapphire, are very common and they cost only a fraction of the natural stones. Smaller synthetic diamonds have been manufactured in large quantities as industrial abrasives for many years. Only recently, larger synthetic diamonds of gemstone quality, especially of the colored variety, have been manufactured.

See also

Notes

  1. ^ The Oxford Dictionary Online and Webster Online Dictionary
  2. ^ Williams, Gardner (1905). The Diamond Mines of South Africa Vol.I and II. Buck & Company. 
  3. ^ Gregory, Theodore (1962). Ernest Oppenheimer. Oxford University Press. 
  4. ^ Burnham, S.M. (1868). Precious Stones in Nature, Art and Literature. Bradlee Whidden.  Page 251
  5. ^ a b c d e Secrets of the Gem Trade; The Connoisseur's Guide to Precious Gemstones Richard W Wise, Brunswick House Press, Lenox, Massachutes., 2003 URL: Secrets of the Gem Trade, Official Website (Has several chapters online)
  6. ^ a b c Church, A.H. (Professor at Royal Academy of Arts in London) (1905). Precious Stones considered in their scientific and artistic relations. His Majesty's Stationary Office, Wyman & Sons.  Chapter 1, Page 9: Definition of Precious Stones URL: Definition of Precious Stones
  7. ^ [Rapaport report of ICA Gemstone Conferene in Dubai]
  8. ^ [Richard Wise Blog on Christie's sale of Padparadscha Sapphire]
  9. ^ Introduction to Lapidary by Pansy D. Kraus
  10. ^ Faceting For Amateurs by Glen and Martha Vargas
  11. ^ Gemstone Enhancement: History, Science and State of the Art by Kurt Nassau
  12. ^ Nassau, Kurt (1994). Gem Enhancements. Butterworth Heineman. 
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Gemstone". A list of authors is available in Wikipedia.
Your browser is not current. Microsoft Internet Explorer 6.0 does not support some functions on Chemie.DE