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TYPES OF GEMSTONES AMETHYST
Amethyst is a form of quartz, with the characteristic purple color ranging from light lilac to dark purple. The color results from small amounts of iron or manganese impurities in the crystal. The darker specimens mined in Africa and Australia are generally considered the most desirable. The relatively low cost and beautiful color make amethyst a perennial favorite for jewelry. Amethyst is manufactured in a wide range of calibrated sizes and shapes. Amethyst can fade after prolonged exposure to sunlight, so it should be stored accordingly.
AQUAMARINE Aquamarine is a blue to blue-green variety of the beryl. Unlike emerald, the green form of beryl, emerald, aquamarine gems are often completely flawless. Aquamarine occurs in relatively large masses, with some specimens weighing in the thousands of carats. The darker the blue color the more valued the specimen, and stones with a greenish tinge are generally less expensive. Most aquamarine sold today is heat treated to improve the blue color. Since aquamarine is so widely available, synthetic material is cost-prohibitive to produce and is not found on the market. Synthetic blue spinel is sometimes sold as aquamarine. Blue topaz is also difficult to distinguish from aquamarine. Both materials are less valuable than real aquamarine, so this gem should be purchased from a reputable dealer.
Aquamarine can fade after prolonged exposure to sunlight and your stone
should be stored accordingly.
CITRINE
Citrine is a form of quartz with a yellow to gold to orange brown hue. Citrine is mined naturally, but most is created by heat-treating amethyst. Heat treated citrine will have a slight reddish tint not found in entirely natural specimens. The availability of citrine means that it is quite affordable and manufactured in a broad range of calibrated shapes and sizes.Like amethyst, citrine will fade after prolonged exposure to sunlight.
EMERALD Emerald is the green form of the mineral beryl. The most important factor in choosing an emerald is the color. As a general rule, the more vivid the green, the more valuable the stone. Emerald are notorious for inclusions, called "jardins," and flawless stones are extremely rare. This is such a common trait that some buyers are suspicious of stones without inclusions since they are considered more likely to be synthetic. Because inclusions are expected, they do not detract from value to the same degree as with other stones, unless they are so deep as to weaken the stone. Inclusions are traditionally filled with oil to mask their appearance, although newer methods have also been developed. Emeralds may also develop internal cracks if struck or subjected to sudden temperature changes.
GARNET
Garnets are not a single type of gemstone, but a group of mineral forms with similar chemical and physical properties. While garnets are commonly thought of as red, they occur in a almost every every color except blue. "Color changing" garnets which have a differing appearance depending on the light source are also mined. Six types of garnet are used as gems: pyrope; almandine, spessartine, grossular, andradite; and uvarovite.
OPAL Opal is a multi-colored gem prized for the irridescent color flashes it displays when viewed from different angles. These flashes of color, known as "opalescence," determine the value of an opal. Opals with demonstrate this "play of color" are referred to as precious opals. Opals with little or no play of color are called common opals. Black opal is the most valued of the precious opals. This form may have a black, dark blue, brown, or very dark green background. White opals, which range in color from white to cream to yellowish background, are the next most treasured members of the family. A third common variety is the fire opal. Fire opals have a transparent or translucent orange to orange-red color. Opals are unique among gemstones in that their brilliance results from the presence of water molecules, rather than trace metals, within the crystal structure. For this reason opals can be damaged by sudden changes in temperature, or by prolonged exposure to strong light or high temperatures.
PERIDOT Peridot, the gem form of the mineral olivine, takes its characteristic yellow-green to olive-green color from iron contained in the crystal structure. Peridot is is treasured as the goddess Pele's tears in Hawaiian culture, and deposits of the mineral are often associated with volcanic activity. .
RUBY The name ruby is reserved for red forms of corundum, the same mineral called sapphire or fancy sapphire in other colors. Transparent, intensely colored ruby has been among the most precious stones for centuries, and top-quality rubies can still sell for more than comparably sized diamonds. A unique form of ruby displays a six-point star pattern, called asterism, caused by light reflections from tiny rutile needles oriented along the crystal faces. Twelve-point stars also occur infrequently. "Star rubies" are often shaped into smooth cabochon cuts to best display this effect. When viewed under a favorable light source the star seems to move across the stone as the the lighting angle changes. .
SAPPHIRE Sapphire is a variety of the mineral corundum. It can appear in almost every color except red. [Actually, red corundum does occur, but it is called ruby rather than red sapphire.] The color blue is so commonly associated with sapphire that stones of any other color are commonly referred to as "fancy sapphires." "Color changing sapphires" are an unusual form which takes on a different appearance depending upon the light conditions. A color changing sapphire may appear blue in under natural light conditions, but take on a violet hue under artificial lights.
SPINEL Spinel, a magnesium aluminate mineral, occurs naturally, in almost any color. The deep red varieties are among the most popular since they are virtually indistinguishable from ruby. In fact, the 170-carat "Black Prince's Ruby" in the British Imperial State Crown was determined to be spinel, but only through sophisticated x-ray diffraction testing. Since spinel is relatively easy to synthesize, you should purchase natural stones, particularly clear stones, only from a reputable dealer. In a strange twist, flawless natural spinel is now found less often than ruby, although the latter remains more valuable.
TOPAZ Topaz occurs in almost all, but the most valuable form is the golden to yellow-orange form known as imperial topaz. Heat treated gems are now widely available in blue and pink forms.
TOURMALINE Tourmaline is available in almost any color, and even occurs with two or even three colors in the same specimen. Like garnet, tourmaline is not a single mineral, but a group of minerals with similar chemical and physical properties. A favorite form is watermelon tourmaline, with the red center surrounded by a green "rind," or vice versa. Because of its long, thin crystal structure tourmalines are commonly shaped as long rectangular bars. Tourmaline is also "pleochroic," meaning that the color changes depending on the angle at which it is viewed. The darkest color will always be seen as you look down the long axis of the crystal.
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