Gemstone
From Wikipedia, the free encyclopedia
A gemstone is a mineral, rock (such in the case of lapis lazuli), or petrified material that when cut or faceted and polished is collectible or can be used in jewelry. Others are organic (such as amber, which is fossilised tree resin, and jet, a form of coal). Some gemstones which may be generally considered precious or beautiful are too soft or too fragile to be used in jewelry (for example, single-crystal rhodochrosite) but are exhibited in museums and are sought by collectors.
Contents |
[edit] 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."
[edit] Value
A gemstone is prized especially for great beauty or perfection so appearance is almost always the most important attribute of gemstones. Characteristics that make a stone beautiful or desirable are colour, unusual optical phenomena within the stone, an interesting inclusion such as a fossil, rarity, and sometimes the form of the natural crystal. 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.
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. In current usage by gemologists, all gems are considered precious, although four of the five original "cardinal gems" (excluding the now-common amethyst) are usually — but not always — the most valuable.
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 iolite.
Gems' costs vary from $500 to $2000(USD). It depends on the type of gem, and how rare it is, such as the expensive diamond.
[edit] 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 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.[1]
Gems which are transparent are normally faceted, a method which shows the optical properties of the stone’s interior to it’s 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 is used to hold the stone onto a flat lap for cutting and polishing the flat facets.[2] Rarely, some cutters use special curved laps to cut and polish curved facets.
[edit] How gems get their color
Gems' color form from light - such as a ruby collects all the colors of the white light-(red, blue, green, etc) and reflects red back to the sun.
Color is the most obvious and attractive feature of gemstones. The color of any material is due to the nature of light itself. Sunlight, 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 eyes 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. - and reflects the red light to the viewer. A colorless stone absorbs none of the light, and so it allows the white light to emerge unchanged.
[edit] 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.[3]
[edit] Heat
Many gemstones are treated by heat, either to improve color or clairity. Most Citrine is made by heating Amethyst and melting amethyst and citrine together makes amethrine. 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 clairity.
[edit] 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 insure public safety. Most greened quartz (Oro Verde) is also irradiated to achieve the yellow-green color.
[edit] Waxing/oiling
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.
[edit] Factors influencing esteem
Factors influencing the esteem in which gems are held are attractiveness, durability, rarity, fashion, light reflection and size, but historical rarity can also play a role, as can the matter of whether there are other kinds of gems offering the same or nearly the same colors. For example, all the quartz gemstones--amethyst, citrines, rock crystal, and so on, are quite inexpensive, but amethyst still enjoys higher esteem than its quartz siblings, both because it was once considered rare and precious, and because it remains the quintessential purple gemstone, while the other varieties of quartz are imitative of more lustrous, harder, and rarer minerals.
[edit] 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.
[edit] See also
[edit] Notes
[edit] External links
Precious metals
Gold • Silver (Sterling silver) • Platinum • Palladium
Gemstones
Diamonds • Rubies • Emeralds • Sapphires • Amethyst • Opals • Aquamarine • Tanzanite • Alexandrite
