Autumn 2020 Gemmology Bulletin Phenomenal Glass Gemstones

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Autumn 2020 Gemmology Bulletin Phenomenal Glass Gemstones

Autumn 2020 Gemmology Bulletin Published: 01 September www.LustreGemmology.com/research By All rights reserved Phenomenal Glass Gemstones By James Evans, FGA At once the most undervalued of gemstones, and the most remarkable… Glass is unrivalled in the range of optical phenomena00 it displays. Goldstone Cat’s-eye Glass (aventurescence) (chatoyancy) This glittering gemstone, also This gemstone displays a- known as ‘Aventurine Glass’, bright and even cat’s-eye. is the origin of the term A more fibrous, shimmering aventurescence. The term was chatoyancy can be seen within later applied to varieties of quartz ‘Victoria Stone’, though the skills and feldspar that contain glittering flakes of mica. required to produce this gemstone no-longer exist. The optical effect is a result of reflections from The cats-eye effect is a result of reflections from the numerous small crystals (typically copper octahedra) core of optical fibres, which are fused together to form which grow within the glass as it cools. a parallel, honeycomb-like structure within the gem. Opalite Uranium Glass (adularescence) (fluorescence) Also called ‘Opalescent Glass’, Also called ‘Vaseline Glass’, this gemstone was favoured this gemstone glows green by Lalique for the golden glow under UV light – a result of the produced by light as it shone uranium it contains. From the through the material, and the cool 1940’s uranium became a strategic blue glow from light shining on and across it. material, leading to a pause in the gem’s production. The optical effect results from the scattering of light as The optical effect results from the chemistry of it hits areas of partial crystallisation within the glass – uranium, not it’s radioactivity: UV light is absorbed by with blue light scattered much more than red. the electrons before being re-emitted as green light. Imitation Zultanite Carnival Glass (colour-change) (iridescence) This remarkable gemstone This vibrant gemstone first displays a vivid green in appeared in the early 20th fluorescent light, yellow in LED Century as an affordable light, an amber-orange in alternative to the ‘Favrile Glass’ incandescent light, and a purplish- jewellery of Louis Comfort Tiffany pink in sunlight. This partially resembles ‘Zultanite’; (son of the famous New York jeweller). a trademarked name for colour-change diaspore. The optical effect results from a thin layer of metal on The optical effect is a result of rare earth elements (Nd the gem’s surface which causes thin-film interference. and Pr) within the glass. A more subtle (lilac to blue) Metal films are also responsible for the play-of-colour effect is produced in ‘Neodymium Glass’. in ‘Slocum Stone’ and pleochroism in ‘Dichroic Glass’..

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Pezzottaite from Ambatovita, Madagascar: a New Gem Mineral
PEZZOTTAITE FROM AMBATOVITA, MADAGASCAR: A NEW GEM MINERAL Brendan M. Laurs, William B. (Skip) Simmons, George R. Rossman, Elizabeth P. Quinn, Shane F. McClure, Adi Peretti, Thomas Armbruster, Frank C. Hawthorne, Alexander U. Falster, Detlef Günther, Mark A. Cooper, and Bernard Grobéty Pezzottaite, ideally Cs(Be2Li)Al2Si6O18, is a new gem mineral that is the Cs,Li–rich member of the beryl group. It was discovered in November 2002 in a granitic pegmatite near Ambatovita in cen- tral Madagascar. Only a few dozen kilograms of gem rough were mined, and the deposit appears nearly exhausted. The limited number of transparent faceted stones and cat’s-eye cabochons that have been cut usually show a deep purplish pink color. Pezzottaite is distinguished from beryl by its higher refractive indices (typically no=1.615–1.619 and ne=1.607–1.610) and specific gravity values (typically 3.09–3.11). In addition, the new mineral’s infrared and Raman spectra, as well as its X-ray diffraction pattern, are distinctive, while the visible spectrum recorded with the spec- trophotometer is similar to that of morganite. The color is probably caused by radiation-induced color centers involving Mn3+. eginning with the 2003 Tucson gem shows, (Be3Sc2Si6O18; Armbruster et al., 1995), and stoppaniite cesium-rich “beryl” from Ambatovita, (Be3Fe2Si6O18; Ferraris et al., 1998; Della Ventura et Madagascar, created excitement among gem al., 2000). Pezzottaite, which is rhombohedral, is Bcollectors and connoisseurs due to its deep purplish not a Cs-rich beryl but rather a new mineral species pink color (figure 1) and the attractive chatoyancy that is closely related to beryl.
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GEM FOCUS August 2020 GOLDSTONE, SUNSTONE, AVENTURESCENCE; A SPARKLING ENIGMA OF CENTURIES... emstones with unusual optical effects are know as sunstone feldspar today. Goldstone is cre - defined as phenomenal gems. Their unusual ated by mixing microscopic copper platelets into Geffect can designate a variety within a molten glass. The sheen and earthy color of this species and add value. i.e., star sapphire. Synthetic material was ideal for Italian mosaics and more counterparts or simulants are often seen, especially affordable jewelry, whereas much larger blocks when the natural counterpart is particularly valuable were used to create ornamental objects such as such as natural alexandrite and color change syn - snuff bottles, small bowls or carved figurines. A thetic sapphire. Aventurescence is a phenomenon common story to its origin purports that the Italian caused by light reflecting micro platelets in a gem glass producers discovered this particular effect by and is easily recognizable from the glittering sheen- accident, but when this was is not clear. Accounts like effect. While green mica crystals in quartz cre - of the first production date vary from one source to ates aventurine quartz, hematite and ilmenite crys - another and are stated as from 11th century to 13th tals create the same effect in sunstone feldspar. century. It is true that this rather attractive Murano glass with reddish brown speckles was a popular Out of all the phenomena seen in gems, aventures - and expensive glass production in Italy for many cence, especially the yellow-brown type, has been centuries but it is hard to believe that sunstone the subject of a long-standing discussion.
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