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|Gemstone, Gem stones|
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. 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 a gemstone as
well. Some minerals that are too soft to be generally
applied in jewelry may still be considered a gemstone
because of their remarkable color, luster or other physical
properties that have aesthetic value. Rarity is another
characteristic that lends value to a gemstone.
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 aluminum 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 luster. 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."
Jewelry made with gem amber 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.
An example of a gemstone for which high prices are determined, not by its rarity, but stability of the market, and also marketing and consumer perception as well, are diamonds.
Diamond is prized highly as a gemstone since it is the hardest naturally occurring substance known and is able to reflect light with fire and sparkle when faceted. However, diamonds are far from rare with millions of carats mined each year.
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 star effects. A factor which plays an important role in determining the value of colored stones, and which is not present in the same way as its determination in diamonds is what many dealers call "water".
Traditionally, common gemstones were classified into precious stones (cardinal gems) and semi-precious stones. The former category was largely determined by a history of ecclesiastical, devotional or ceremonial use and rarity. Only five types of gemstones were considered precious: diamond, ruby, sapphire, emerald, and amethyst. After the discoveries of bulk Amethyst in Brazil in the 19th century, Amethyst lost its place amongst the list of precious stones.
Nowadays such an approach is outdated amongst the present generation of jewelry designers, gemologists and gem dealers. 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 pigeon-blood almost "perfect" Ruby.
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.
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 vary 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.
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.
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 and give a purer blue. Nearly all Tanzanite is heated to 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 high heat 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 contain natural fissures that 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.
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.