Yellow-Green and Green Beryl
Yellow-green beryl has achieved 'desired gem' status with consumers just within the last few years. Museums were eager to acquire spectacular specimens, while connoisseurs sought it for their collections. However, fashion trendsetters ignored it. As in aquamarine, the iron incorporated into its chemical structure is responsible for the color..
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Yellow-green beryl has achieved "desired gem" status with consumers just within the last few years. Museums were eager to acquire spectacular specimens, while connoisseurs sought it for their collections. However, fashion trendsetters ignored it. As in aquamarine, the iron incorporated into its chemical structure is responsible for the color.
In Gemstones of the World, Walter Schumann states that "Green beryl is designated in the trade as aquamarine, as the color can be heat-treated at 752-842 degrees F (400-450 degrees C) and improved into aquamarine." A very interesting report on heat-treatment of green beryls may be found on page 114 of the soft cover volume of Dr. John Sinkankas' Gemstone and Mineral Data Book. He first mentions that a specimen of yellow beryl became a pale blue when heated to 400 degrees C.
Experiments showed that "olive-brown, greenish-yellow, and yellowish-green" materials lost the yellow tones at 250-280 degrees C and turned a clear green. Upon reheating to 280-300 degrees C, the stones changed to blue. The blue tones begin to appear at 280-300 degrees C, with the rate of change increasing as the temperature rises to the optimum 400-450 degrees C. Dr. Sinkankas believes all green beryl becomes blue when heat-treated. He states that "the depth of blue is directly proportional to the depth of the original green" and that "the finest blues are obtained from dark oil-green or dark olive-green specimens." The color is stable to 1025 degrees C. At higher temperatures, the material may become colorless or greenish. Crystals harboring mineral, liquid, or gaseous inclusions are less apt to undergo heat treatment. Different heat-expansion rates of the inclusions and the host crystal could fracture or shatter the crystal.
The major source of green beryl is the state of Minas Gerais in Brazil. Crystals recovered from the hydrothermally-influenced granitic pegmatites there are noted for their numerous gas-liquid inclusions and also for the striking patterns of inclusions developed on partially healed fracture planes. Excellent pictures depicting the examples of these are found on page 158 of Dr. E. Gubelin's Internal World of Gemstones. Deposits in hydrothermal greisen formations (granitic rock composed of quartz, mica, and topaz) in the Transbiakalia region of the former U.S.S.R. yield fine alkali-free crystals with few inclusions. Madagascar, India, and Sri Lanka are the other sources of this variety of beryl.
Beryl has been recognized as an excellent material for gem carvings for centuries. New designs and techniques for faceting gemstones are proliferating. The growing demand for such carvings and "designer" gems has increased the public's knowledge and acceptance of many previously unappreciated gem materials. This includes the beautiful and durable natural green beryls, a distinctive choice for all types of jewelry appropriate for both ladies and gentlemen.
Correction: In the third paragraph of the previous Let's Talk Gemstones article on aquamarine, an error occurred in the transcription of my notes concerning the Marta Rocha crystal. The two sentences should read as follows:
"The standard for comparison of color in aquamarine was set by the deep blue Marta Rocha crystal recovered from the same area. Dr. Joel Arem tells us in his Color Encyclopedia of Gemstones that 300,000 carats of superb gems were cut from this 134 pound crystal."
|Composition:||beryllium aluminum silicate Be 3 Al 2 Si 6 O 18 +Fe|
|Crystal System:||hexagonal per Arem; hexagonal (trigonal) per Schumann|
|Variety:||green, yellow-green, brownish-yellow-green|
|Colors:||green, yellow-green, brownish-yellow-green|
|Phenomena:||chatoyancy and asterism uncommon|
|Diaphaneity:||transparent, translucent to opaque|
|Habit:||prismatic; often striated and etched|
|Specific Gravity||varies from 2.67 to 2.71|
|Hardness||7.50 to 8.0|
|Toughness:||good; sometimes brittle|
|Refractive Index||o=1.567 to 1.583; e= 1.572 to 1.590|
|Birefringence:||varies from 0.005 to 0.007|
|Optic Character||uniaxial negative|
|Pleochroism||definite yellow-green/blue-green per Schumann|
|Ultraviolet Fluorescence||inert; iron quenches fluorescence|
|Spectra||no definite references found|
|Color Filter||no information|
|Aqua Filter||specimen from private collection: definite greenish-blue|
|Chelsea Filter||specimen from private collection: definite greenish-blue|
|Solubility||insoluble except in fluoric acid|
|Thermal Traits||avoid thermal shock; remove stone during jewelry repairs|
|Inclusions||gas-liquid; patterns on partially healed fracture planes|
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