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Milk and Honey
Some gems with a highly developed chatoyance can appear to be made of two different materials when illuminated from the proper direction with respect to the observer’s eye. In these stones, the cat’s-eye will appear to divide the stone into a zone of light-colored material on one side of the eye and dark-colored material on…
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Many Types of Gems Exhibit Chatoyance
Many types of gems exhibit chatoyance; however, it is not present in every specimen of those gems and is usually only seen in a minority of specimens. Gems with well-known chatoyance include the following: chrysoberyl, tiger’s-eye, actinolite, apatite, beryl (aquamarine, heliodor, emerald), beryllonite, cerussite, danburite, diaspore, diopside, enstatite, garnet, iolite, kyanite, moonstone, opal, peridot, pezzottaite, prehnite, quartz, rutile, sillimanite, scapolite, spinel, topaz, tourmaline, and zircon.
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Understanding Chatoyance
A good analogy for the chatoyance effect is how light reflects to form a line across the surface of a spool of silk thread. Viewing and moving a spool of silk thread under a beam of incident light can be a useful way to develop an understanding of how parallel inclusions produce an “eye” within…
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Chatoyance – Eye of the Cat
The name “chatoyance” originates from the French word “chatoyer,” which means “to shine like a cat’s eye.” The analogy also matches the way a cat’s pupils will narrow to a thin slit under bright light. Cat’s Eye with a Slit Pupil: This cat’s eyes have responded to bright light by closing their pupils to a narrow…
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What is Chatoyance?
Chatoyance is an optical phenomenon in which a band of reflected light, known as a “cat’s-eye,” moves just beneath the surface of a cabochon-cut gemstone. Chrysoberyl and tiger’s-eye are two of the best-known gem materials that exhibit this phenomenon. Excellent specimens of chrysoberyl exhibit the finest chatoyance, and tiger’s-eye is the chatoyant gem most widely used in jewelry. Chatoyance occurs…
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Ant Hill Garnets as Diamond Indicators
Ants delivering small mineral particles to the surface is not unique to Arizona. It is known in many parts of the world. In a few instances, ant hill minerals have been used as a prospecting tool. In prospecting for diamonds, geologists use “indicator minerals” to determine if they are on or near a kimberlite pipe – the host rock of…
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Arizona Ant Hills
A few areas in Arizona are well known for ant hill garnets. These are beautiful bright red chromium pyrope garnets with a very high color saturation. The Native Americans discovered them long ago and marveled at their color and beauty. They regarded them as special and sometimes sewed them into ceremonial rattles or gave them…
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What are Ant Hill Garnets?
Some gemstones derive a large portion of their appeal because they are found in an unexpected locality or have an unusual origin. “Ant hill garnets” are one of the more interesting of these “novelty gems.” They are called “ant hill garnets” because they are found on and around the margins of ant hills. The ants encounter the garnets while…
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Sources of Aquamarine
Brazil is the world’s leading source of gem-quality aquamarine for over 100 years. It is found there in both pegmatites and alluvial deposits. Afghanistan, Kenya, Madagascar, Malawi, Mozambique, Nigeria, Pakistan, Russia, Tanzania, and Zambia are all sources of aquamarine. India and Sri Lanka have long histories of aquamarine production from alluvial deposits. Aquamarine has been found at many locations in the United States. To date, almost all of the production has been…
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Geological Occurrence
Most gem-quality aquamarine crystals form in cavities below Earth’s surface during the late stages of a magma body’s crystallization. The cavities provide open space for crystals to grow. The slow underground cooling rate provides the time needed for the development of large crystals. And, the late-stage timing provides the fluids and a supply of the beryllium needed…