Gemstone Varieties & Characteristics
Of over 4,000 identified minerals, only around 70 are classified as gemstones. These stones, along with a few synthetic compounds, are prized primarily for their rarity and beauty. There are several different ways to classify gemstones, from the classic precious and semi-precious divisions to more modern structural classifications. Gemologists and jewelers also use various descriptors for gems, such as lustre, that measure gem qualities and may impact a gemstone's value. Does this Spark an idea?
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Grading and Characteristics
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Gemstones are graded by several characteristics, most frequently those of hardness, clarity, purity and color. There are several other characteristics used to grade specific gemstones, like their interaction with light or magnetic properties. Gemologists set specific ideal characteristics for each gemstone as guidelines by which to measure stones. Examples of measurement scales include the Mohs hardness scale, which measures stone hardness from 1-soft to 10-hard, and the GIA type and clarity scale, which measures how pure the uncut, untreated stone is by the level of expected inclusions (impurities).
The Four Precious Stones
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All gemstones were first classified as precious or semi-precious. The classic precious stones were diamond, emerald, sapphire and ruby, with pearl occasionally included. These stones were considered precious because at the time they were rare and highly prized, and most nations taxed them at a higher rate than other stones. Today, a number of gemstones, including synthetics, are rarer and more expensive than the classic precious stones, but in general the original four or five are still referred to as the precious gems.
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Mineral, Organic and Synthetic
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The process involved in a stone's creation is an excellent way to classify it. Natural gemstones may be either mineral or organic in origin. Organic stones, like jade, jet and pearl, are formed from the remains of prehistoric plants and animals. Organics are never crystalline, though they may be translucent. Mineral stones are formed by geological processes and include the four original precious stones, as well as many other stones from turquoise to tourmaline. Mineral stones may be further subdivided into crystal, which are generally cut into finished products, and amorphous, which are generally polished. Synthetic stones are man-made and may either be previously-unknown gemstones or mimic the natural stones, as in synthetic diamond.
Quality of Light
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The effects of light on gemstones help not only divide them into varieties but determine the relative quality of each. Stones may be measured by lustre, scintillation, brilliance and fire. Scintillation and brilliance are only applied to crystalline stones and measure, respectively, the play of light when viewed and the amount of light reflected from the stone's center. Lustre may be applied to any gemstone and measures the amount of light reflected from the stone's surface. Fire measures the prismatic splitting of light into its component colors and is perhaps the most unique measurement. Diamond, for instance, can split light into a rainbow just as a prism might. Opal is classified by fire because light splits on the surface of the stone into component colors, transforming a pearlescent stone into one displaying a rainbow of colors along fracture points.
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References
Resources
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