Gemstone
A gemstone or gem (also called a precious or semi-precious stone, a fine gem, or jewel) is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments.[1][2] However certain rocks, (such as lapis lazuli) and organic materials (such as amber or jet) are not minerals, but are still used for jewelry, and are therefore often considered to be gemstones as well. Most gemstones are hard, but some soft minerals are used in jewelry because of their lustre or other physical properties that have aesthetic value. Rarity is another characteristic that lends value to a gemstone. Apart from jewelry, from earliest antiquity until the 19th century engraved gems and hardstone carvings such as cups were major luxury art forms; the carvings of Carl Fabergé were the last significant works in this tradition.
Characteristics and classification
The traditional classification in the West, which goes back to the
Ancient Greeks, begins with a distinction between
precious and
semi-precious stones; similar distinctions are made in other cultures. In modern usage the precious stones are
diamond,
ruby,
sapphire and
emerald, with all other gemstones being semi-precious.
[3] This distinction reflects the rarity of the respective stones in ancient times, as well as their quality: all are
translucent with fine color in their purest forms, except for the colorless diamond, and very hard,
[4] with hardnesses of 8–10 on the
Mohs scale. Other stones are classified by their color,
translucency and hardness. The traditional distinction does not necessarily reflect modern values, for example, while
garnets are relatively inexpensive, a green garnet called
Tsavorite, can be far more valuable than a mid-quality emerald.
[5] Another unscientific term for semi-precious gemstones used in
art history and
archaeology is
hardstone. Use of the terms 'precious' and 'semi-precious' in a commercial context is, arguably, misleading in that it deceptively implies certain stones are intrinsically more valuable than others, which is not the case.
In modern times gemstones are identified by
gemologists, who describe gems and their characteristics using
technical terminology specific to the field of
gemology. The first characteristic a gemologist uses to identify a gemstone is its
chemical composition. For example, diamonds are made of
carbon (C) and rubies of
aluminium oxide (Al
2O
3). Next, many gems are crystals which are classified by their
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.
Gemstones 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),
red beryl (red),
goshenite (colorless),
heliodor (yellow), and
morganite (pink) are all varieties of the mineral species
beryl.
Gems are characterized in terms of
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.
Gemstones may also be classified in terms of their "water". This is a recognized grading of the gem's luster and/or transparency and/or "brilliance".
[6] Very transparent gems are considered "
first water", while "second" or "third water" gems are those of a lesser transparency.
Value
There is no universally accepted grading system for gemstones. Diamonds are graded using a system developed by the
Gemological Institute of America (GIA) in the early 1950s. Historically, all gemstones were graded using the naked eye. The GIA system included a major innovation: the introduction of 10x magnification as the standard for grading clarity. Other gemstones are still graded using the naked eye (assuming 20/20 vision).
[8]
A
mnemonic device, the "four Cs" (color, cut, clarity and carats), has been introduced to help the consumer understand the factors used to grade a diamond.
[9] With modification, these categories can be useful in understanding the grading of all gemstones. The four criteria carry different weight depending upon whether they are applied to colored gemstones or to colorless diamond. In diamonds, cut is the primary determinant of value, followed by clarity and color. Diamonds are meant to sparkle, to break down light into its constituent rainbow colors (dispersion), chop it up into bright little pieces (scintillation), and deliver it to the eye (brilliance). In its rough crystalline form, a diamond will do none of these things; it requires proper fashioning and this is called "cut". In gemstones that have color, including colored diamonds, it is the purity and beauty of that color that is the primary determinant of quality.
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 Helen of Troy was known to have worn star-
corundum.
[10]
Historically, gemstones were classified into
precious stones and
semi-precious stones. Because such a definition can change over time and vary with culture, it has always been a difficult matter to determine what constitutes
precious stones.
[11]
Aside from the
diamond, the
ruby,
sapphire,
emerald,
pearl (strictly speaking not a gemstone) and
opal[11] have also been considered to be precious. 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 been popular and hence been regarded as precious.
Nowadays such a distinction is no longer made by the trade.
[12] 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
red beryl.
Gem 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 sizes of stone can affect prices. Typically prices can range from 1USD/carat for a normal amethyst to US$20,000–50,000 for a collector's three carat pigeon-blood almost "perfect" ruby.
Ruby 
A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum (aluminium oxide). The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires. The ruby is considered one of the four precious stones, together with the sapphire, the emerald, and the diamond.
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| Ruby Jewelry |
Prices of rubies are primarily determined by color. The brightest and most valuable "red" called pigeon blood-red, commands a huge premium over other rubies of similar quality. After color follows clarity: similar to diamonds, a clear stone will command a premium, but a ruby without any needle-like rutile inclusions may indicate that the stone has been treated. Cut and carat (weight) are also an important factor in determining the price.
Sapphire
Sapphire (
Greek:
σάπφειρος;
sappheiros, "blue stone"
[1]) is a
gemstone variety of the mineral
corundum, an
aluminium oxide (α-
Al2O3), when it is a color other than red or dark pink; in which case the gem would instead be called a
ruby, considered to be a different
gemstone. Trace amounts of other elements such as
iron,
titanium, or
chromium can give corundum blue, yellow, pink, purple, orange, or greenish color. Pure chromium is the distinct impurity of rubies. However, a combination of e.g. chromium and titanium can give a sapphire a color distinct from red.
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| Pink Sapphire |
Blue sapphire
Color in gemstones breaks down into three components:
hue,
saturation, and
tone. Hue is most commonly understood as the "
color" of the gemstone. Saturation refers to the vividness or brightness or "colorfulness" of the hue, and tone is the lightness to darkness of the hue.
Blue sapphire exists in various mixtures of its primary (blue) and secondary hues, various tonal levels (shades) and at various levels of saturation (brightness).
Blue sapphires are evaluated based upon the purity of their primary hue.
Purple,
violet, and
green are the most common secondary hues found in blue sapphires. Violet and purple can contribute to the overall beauty of the color, while green is considered to be distinctly negative. Blue sapphires with up to 15% violet or purple are generally said to be of fine quality. Blue sapphires with any amount of green as a secondary hue are not considered to be fine quality. Gray is the normal saturation modifier or mask found in blue sapphires. Gray reduces the saturation or brightness of the hue and therefore has a distinctly negative effect.
The color of fine blue sapphires can be described as a vivid medium dark violet to purplish blue where the primary blue hue is at least 85% and the secondary hue no more than 15% without the least admixture of a green secondary hue or a gray mask.
Yellow and green sapphires are also commonly found. Pink sapphires deepen in color as the quantity of
chromium increases. The deeper the pink color the higher their monetary value as long as the color is trending towards the red of rubies.
Sapphires also occur in shades of orange and brown, and colorless sapphires are sometimes used as diamond substitutes in jewelry. Padmaraga sapphires often draw higher prices than many of even the finest blue sapphires. Recently, more sapphires of this color have appeared on the market as a result of a new artificial treatment method that is called "lattice diffusion.
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.
Moissanite, also a synthetic stone, is another example. The imitations copy the look and color of the real stone but possess neither their chemical nor physical characteristics. Moissanite actually has a higher refractive index than diamond and when presented beside an equivalently sized and cut diamond will have more "fire" than the diamond.
However, lab created gemstones are not imitations. For example, diamonds,
ruby,
sapphires and
emeralds have been manufactured in labs to possess identical chemical and physical characteristics to the naturally occurring variety. Synthetic (lab created)
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, although larger gem-quality synthetic diamonds are becoming available in multiple carats.
[18]Whether a gemstone is a natural stone or a lab-created (synthetic) stone, the characteristics of each are the same. Lab-created stones tend to have a more vivid color to them, as impurities are not present in a lab and do not modify the clarity or color of the stone.
( read more the completed articles at http://en.wikipedia.org/wiki/Gemstone )
