A gemstone usually refers to a rock or mineral that is used in jewelry or as a collector's item, usually after it has been polished, cut or faceted.
Gemstones are classified through several ways. They may be grouped according to chemical composition -- for example, rubies are made of aluminum oxide and diamonds are made of carbon. If they are crystals, they may be classified using the crystal system, which divide gemstones into cubic, trigonal or monoclinic. They are also classified according to habit, which refers to the shape of the stones. It's also possible to group gemstones according to species -- for example, the mineral species beryl would include such gemstones as emerald, aquamarine, bixbite, goshenite, heliodor and morganite.
Buying a gemstone can be an absolute joy. Imagine the thrill of being surrounded by a roomful of precious gems, with colors that dazzle and captivate. Over 30 types of gem varieties have been identified, not counting those rare, singular gemstones that show up once in a while and defy category. Some of these gemstones have decorated and adorned the earliest civilizations of man, while others are recent discoveries but nonetheless as impressive as their forebears.
Emeralds are the most popular gemstones in the world and have been esteemed since ancient times. They are a dazzling green all over, alternately shiny and intense to the eye. The highest-quality emeralds are more coveted than diamonds, a little known fact. They were named from the word "esmeralde" which is Old French for "green gemstone." Their rich history dates back to the times of great ancient civilizations like the Incas and Aztecs. Its roots are buried deep in the jungles of South America where the rarest of emeralds can still be found.
The ruby is the gemstone that best represents love and vitality, passion and intensity. The ruby has long been regarded as first among gemstones, their bright red glow that conveys strength and warmth. These qualities made the ruby the most valuable gemstone for centuries, a royal favorite of emperors and kings.
Often compared to the blue sky in literature and history, sapphire has as many shades as the sky has hues. Legend has it that early man actually believed that the sky was made of one big block of sapphire and that the earth was simply a thorn on its side. Among emotions, Sapphire is associated with noble feelings of harmony, friendship, loyalty and sympathy. And since blue is the favorite color of about half the world's population, sapphire's appeal and popularity worldwide is not surprising.
In the last two decades there has been a proliferation of certification, not only for diamonds but for gemstones as well. There are five  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. 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.)
Gem dealers are fully aware of the differences between gem laboratories and will make use of the discrepancies to obtain the best possible cert. 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 if the stone were from Burma.
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.
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. 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. Rarely, some cutters use special curved laps to cut and polish curved facets.
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.
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.  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.
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.
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 has been in use with different gemstones such as Diamonds, Emeralds, 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.
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.
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, which 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.