He saw how much people loved colored flowers, so he colored gems to gain power and control over mankind. The facts, however, are less fanciful. Of the beautifully crystallized minerals that seem useful for gems, only a very few actually meet the standards, that is, are sufficiently beautiful, durable, rare, and large enough to be cut into salable stones.
As a class of natural objects, gemstones are exceedingly rare. About one hundred chemical elements make up the earth. In gemstones, they are major ingredients in amethyst, aquamarine, emerald, garnet, peridot, topaz, tourmaline, and zircon. Oxygen is a major ingredient in ruby, sapphire, chrysoberyl, and spinel. As a mineral forms, certain atoms attract each other and arrange themselves in an orderly geometric pattern called the crystal structure.
All mineral crystals have their atoms arranged in some combination of fourteen basic patterns. Minerals usually occur as crystalline grains in rocks. Time is another important factor in crystal growth. When molten rock cools quickly, natural glass or tiny crystals form. Slower cooling time gives larger crystals time to grow. Large crystals may form whenever conditions are right. They may grow slowly into open spaces in cracks or hollows in the rocks.
Occasionally, nearly perfect crystals are found. Crystal shape often helps identify and distinguish gem minerals from one another. Today, many gems can be creates in laboratories. Synthetic gems have the same chemical composition and physical properties as naturally formed gemstones. A simulated gem may look like a natural gem, but there the similarity ends.
As with other gems, most precious gemstones are minerals. This mineral, however, is a chemical element or compound that forms in nature and possess a unique internal atomic structure, crystal. Minerals usually form as a result of inorganic processess that occur in rocks. Furthermore, since mineral deposits can be found all over the world, so can the various gemstones.
Of some interest is the mining techniques used to procure such brilliant gems. For instance, the mining of opals in Australia is most enlightening. The opal miner there is a strange breed of individual. He chooses to lead a spartan life in a particularly barren and dry corner of the world while he searches for his rainbows. To escape the extreme temperatures, he must burrow a home underground. Since deposits are spead over a wide area, there is little clue to their location.
Mining is done on a small scale with hand-operated machinery and small tools. A pocket knife might be the final instrument to loosen an opal from its host rock. Zambia, Tanzania, Norway, and India. India, Madagascar, Sri Lanka, Africa,. Australia, and China Garnet iridescent andradite Mexico. Opal black Australia. The marketing mix of precious stones is widely varied. Gem stones differ vastly from color to size to clarity. In short, no two stones are identical, each is unique and therefore hard to standardize.
Following is a summary of the types of products available, issues of pricing, availability at retail, and promotional efforts which support the uniqueness of these colored stones.
To be considered a gemstone, a material must be attractive enough to be used for personal adornment. Usually the term refers to minerals that have been cut and polished.
Beauty is the common quality all gemstones share. Images of perfection, wealth, and status also surround gemstones. Some of the most precious colored gems are ruby, emerald, and sapphire.
These stones as well as numerous others, such as aquamarine, amethyst, garnet, opal, peridot, and turquoise, are found all over the world. Ruby and sapphire are gem varieties of the mineral corundum. Pure corundum is colorless aluminum oxide. It is widespread in small amounts in metamorphic rocks. Its gems are among the most durable and can be safely worn in rings.
Because corundum is denser than diamond, a one-carat sapphire or ruby will be smaller than a one-carat diamond. Red corundum, ruby is one of the rarest and most costly gemstones. A bit of chromium substituting for the aluminum in the crystal structure colors ruby red. All other colors of gem corundum are called sapphire. Emerald and aquamarine are gem varieties of the mineral beryl.
Beryl is beryllium aluminum silicate. It is a major source of beryllium, a lightweight metal similar to aluminum. Beryl usually occurs in granitic rocks. Green gem beryl, or emerald, usually contain inclusions and fractures that make them somewhat fragile.
They should be mounted and worn with care. Jewels are most often cut in a characteristic step-cut rectangle or square. Bauxite is a sedimentary rock that is an important ore of aluminum. The aluminum content in it is leeched from the soil above. Cobalt is famous for the incredible blue color it imparts to glass and pigment.
It has been found in meteorites and is used in invisible ink. It is a brittle metal and resembles iron. Fluorite fluorspar is commonly used to create fluorescent pigment and since it is very beautiful, it is used for gem material. It is mined all over the world.
Gold is the most familiar metal to most people. It is used for jewelry, dentistry, electronics and a host of other applications. It is the most malleable metal which increases the way it can be used. Halite [image right] sodium chloride--salt is used for seasoning food and softening water. It is also used to make certain acids, in fire extinguishers and melting ice on the road. Iron Ore is perhaps as important to civilization today as gold historically has been.
It is used in all sorts of construction from vehicles to buildings. Lead has a bad reputation for its poisoning capabilities, some of which may have been exaggerated by fear. It cannot be absorbed by the skin or breathing, but it is harmful if it touches food or drink.
It was at one time used in paint, pencils and eating utensils. Lithium is used in several applications including medication for bipolar symptoms and batteries. Lithium has become very popular with the advent of electric cars.
Manganese with iron impurities can be slightly magnetic. It is essential in the steel making process, and petroglyphs were carved into it in the Southwest. Mica is the mineral responsible for putting a sparkle on many rocks. This mineral is very flexible, and large sheets of it were used as window glass in the past.
Nickel is a common metal in everyday life. It has been used in currency, jewelry and eating utensils and is used in alloys as well. Potash is the old fashioned term for Potassium. Potassium is a major component in crop fertilizer around the world. It is also used in soap manufacture. Native Americans polished it to use as a mirror, and it is occasionally used in jewelry. Its byproduct is used in ink and disinfectants. Quartz [image left] silica is the most abundant mineral on earth.
It is the name for a large family of rocks including the jaspers, agates, onyxes and flints. Quartz is used in concrete, glass, scientific instruments and watches. Most importantly today, it is used to make silicon semiconductors. Silica is used in desiccants to remove moisture from the air.
It is also used in sandpaper and glass making. Rare Earth Elements lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium ytterbium and lutetium Many of these are used to create nuclear power. Silver is one of the precious metals. Diamonds are divided into types according to the presence or absence of nitrogen and boron, as well as the structural organization of these impurities within the crystal lattice.
Type I diamonds are described as containing significant nitrogen that is detectable by infrared absorption spectroscopy a process that detects which wavelengths are absorbed or transmitted by a stone, each element being associated with typical wavelengths. Type II diamonds do not contain significant nitrogen. Color in natural diamond is related primarily to the substitution of other elements for nitrogen and other defects caused by physical deformation in the crystal lattice; there are often multiple color-causing defects in a single sample.
Type I diamonds in which the nitrogen impurities are clustered are generally colorless, brown or yellow; when the impurities are more widely diffused the diamonds are yellow, orange or brown. Pink, red and purple diamonds are also of Type I, and the coloration has been tied to deformation of the impurity-laden part of the crystal lattice after the gem finished forming. Type II diamonds contain very few or no nitrogen impurities but may have boron impurities, which typically render the diamond blue to gray.
When they are virtually devoid of all impurities, they are colorless or brown. Mineral inclusions within diamonds permit calculation of pressures and temperatures of the environment in which they formed. Diamonds generally crystallize at depths of to kilometers and at temperatures of 1, to 1, degrees Celsius.
The vast majority originates from within the lithosphere the rigid crust and upper mantle of the Earth below very old, stable parts of the continental crust called cratons —areas toward the center of tectonic plates that are far from areas of growth or subduction. The rest originate largely from sublithospheric sources, which can be as deep as the lower mantle.
Such sources are generally described as being within deep keels of ancient cratons, where geothermal energy is suppressed by these relatively cold masses, thus allowing crystallization to occur. This setting is low in silica, and is dominated by rocks such as peridotite or eclogite, both of which are high in magnesium and iron. Well-formed diamond crystals most likely result from two processes. One is the reduction the gain of electrons of oxidized carbonate CO 3 in its solid state, or dissolved within a melted rock or chemical-rich fluid.
The other is the oxidation the loss of electrons of reduced carbon in the form of methane. Crystallization in either a molten-rock or a fluid-dominated setting allows physically unconstrained crystal growth.
The carbonate component has been hypothesized to originate from carbon introduced into the mantle when volatile chemicals, such as CO 2 , escape oceanic crust as it subducts beneath another tectonic plate and enter a region of molten rock.
Methane that is oxidized to diamond is thought to originate from reduced fluids in the upper mantle. The release of water by this reaction would aid in fluid-driven, or metasomatic , reactions at the site of diamond growth in the subcratonic lithosphere.
The ultimate origin of carbon in diamonds, however, is ambiguous and a subject of ongoing research. Models for mantle evolution suggest that diamonds older than 2. Radiometric dating determines age by measuring the percentage of different isotopes variants of elements that differ only in the number of neutrons in a material that has naturally decayed over geologic time. Such analysis of diamond is achieved by studying its silicate and sulfide mineral inclusions that crystallized at the same time as the host diamond.
However, analyses of minerals along exposed planes where the crystal has cleaved have been shown to record the date of the eruption that brought the diamond to the surface, not necessarily the date of its formation. Figure 3. The kimberlite pipes that delivered diamonds to the surface, and the types of rock they have traveled through, have been mapped for five mines in Kimberley, South Africa.
The diatreme zone is where diamonds can be found. Image adapted from M. Field et al. Regardless of depth of formation, diamonds are transported quickly to the surface via rapidly rising bodies of molten rock—called kimberlite or lamproite magmas—that originate from the growth areas themselves or from greater depths. The transporting magma may be corrosive to diamond and thus requires a speedy ascent to preserve the gems. Kimberlites erupt at average speeds of 10 to 30 kilometers per hour through the rapid release of carbon dioxide and water, creating buoyancy.
However, the mechanism of this release was not clear until recent work by James K. Russell and his colleagues at the University of British Columbia. An increase in silica content in the magma causes a quick drop in its solubility of carbon dioxide, causing continuous and vigorous expulsion of the gas and driving the ascent of the kimberlitic magma.
Kimberlites are usually about 65 to million years old, but some are as old as 1. They are not as aged, however, as the diamonds they end up transporting. With few exceptions, diamond formed well before kimberlite or lamproite eruption—on the order of hundreds of millions to billions of years.
Although eruption ages of diamondiferous kimberlite are variable, those kimberlites younger than 1. Diamond deposits older than about 1. Studies on the origin of host structures and global distribution of the gemstone have facilitated research and understanding of the deep Earth and led to various methods for laboratory synthesis of diamonds.
Ruby and sapphire are gem varieties of the mineral corundum, essentially an oxide of aluminum that has the general formula Al 2 O 3. They can command some of the highest prices paid for any gem: In an 8.
Figure 4. Uncut natural ruby crystals, about 2 centimeters long, from Winza, Tanzania. These are caused by light reflecting from needle-like inclusions of a titanium-oxide mineral called rutile, or other iron or iron-titanium oxide phases, aligned along crystallographic planes and parallel to the hexagonal faces at 60 degrees.
The color of ruby is due to chromium replacing aluminum in the crystal structure. Chromium is also responsible for the green color of emerald, and the reason for the difference in color between emeralds and rubies is still unresolved. In both gems, the chromium is surrounded by six oxygen atoms, but absorbs light differently in each crystal. One theory for the color variation is that it is caused by the electrostatic potential imposed by the rest of the lattice ions on the active electrons of the chromium- oxygen unit.
The main effects are thought to be from the electric field generated in the neighborhood of the chromium-ion site in ruby, which is absent in emerald because of the symmetry of its lattice. This charge results in the absorption features being shifted to higher energies in ruby such that the gem has two large bands of visible light absorbed at wavelengths of approximately and nanometers, and two transmission windows at nanometers blue and nanometers red.
Ruby appears red because the human eye is more sensitive to red above nanometers than to blue. Red fluorescence under ultraviolet light and sometimes daylight, combined with the red color of ruby, is the cause of the fire effect seen in many rubies from Myanmar and Vietnam. Figure 5. Sapphires were discovered by an Inuit hunter on Baffin Island in Canada in top left.
A microphotograph taken with cross-polarized light shows a polished thin section 30 microns thick of sapphire-bearing rock from the same locale top right. The gold in the center is pyroxene; it is rimmed by a mixture of green mica and gray feldspar, which is in turn circled by purple scapolite and gray nepheline.
A method used to find sapphire on Baffin Island is to use ultraviolet light to excite fluorescence from scapolite, as this mineral often occurs with sapphire. However, at this location in the summer, it is only dark enough to use this method for two to four hours a night.
The blue color of sapphire results from electron transfer between less than 0. This charge transfer uses specific amounts of energy from light at certain wavelengths; the wavelength used is absorbed and not seen.
In sapphire, light from the red end of the spectrum is used as energy for the charge transfer between iron and titanium atoms, making the gem look blue. Orange-pink gem corundum is called padparadscha, from the Sanskrit term for the color of the lotus flower.
Most gem corundum is produced from placer deposits that are classified as alluvial water transport , colluvial gravity transport and eluvial weathering. Gem corundum is also produced from paleoplacers. The global distribution of corundum deposits is linked to collision, rift and subduction geodynamics. Three main periods of corundum formation are recognized: the Pan-African orogeny to million years ago , which produced primary gem corundum deposits in Africa, India, Madagascar and Sri Lanka; the Himalayan orogeny 45 to 5 million years ago , which produced the marble-hosted ruby deposits in Asia; and Cenozoic alkali-basalt extrusions 65 to 1 million years ago.
Newer major producers include Australia, Madagascar and Vietnam. The finest rubies and sapphires come from thick marble layers composed of calcite CaCO 3. But how did the aluminum needed to form corundum get into the marble? How about the chromium, titanium and iron needed to impart color?
If silica were present, it would bind with aluminum and prevent the formation of corundum. My colleagues and I are currently studying a ruby and pink-sapphire deposit in central British Columbia to investigate these questions. Preliminary results suggest that the material precursor to the marble was limestone deposited in thin interlayers of mudstone. When the limestone metamorphosed to form marble, the minerals in the mudstone primarily mica, containing silica and many other trace minerals underwent a complicated series of reactions to ultimately form corundum.
Emerald is the green gem variety of the beryllium-based mineral beryl with general formula Be 3 Al 2 Si 6 O Like corundum, beryl crystallizes in the hexagonal system. The color of emerald is due to trace amounts of chromium, vanadium or both elements replacing aluminum in the crystal structure.
In the beryl crystal, rings of silicon and oxygen are stacked, leaving channels in the center that can trap water or other impurities. Some precious stones actually increase in value due to their inclusions.
Star sapphire, for example, derives both its value and its name from its needle-like inclusions. The interplay between hue, saturation, and tone produces the color, and coverage refers to the evenness of that color throughout the gemstone. Almost all colored stones are assessed principally by their color.
In fact, certain colors determine the actual classification of a gem! For instance, rubies and sapphires have similar chemical compositions, but we distinguish them due to their difference in color.
Balancing carat, clarity, and color when cutting a gem is known as the "cutter's tradeoff" because it often means preserving one quality at the expense of another. For instance, cutting an alexandrite to display the best color means sacrificing carat, and conversely, preserving carat instead of removing an inclusion may result in less clarity. In addition to the Four Cs, there are other factors that can affect the value of a gem, including country of origin and whether the gem has been artificially enhanced.
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