Geology of some gemstones Ornamental Materials from Antigua

Geology of some gemstones

Ornamental Materials from Antigua

In the Eifel Mountains, hauyne formed in a magma chamber approximately 2-4 km below the surface, together with a suite of minerals including sanidine, nosean, nepheline, leucite, plagioclase, amphibole, magnetite, titanite, phlogopite, apatite and olivine. This phonolitic magma was volatile-rich and chemically zoned, with a strong decrease in sulfur during progressive magmatic differentiation, which is interpreted to be partially caused by crystallization of hauyne. This magma erupted approximately 12,900 years ago, and the resulting volcanic rocks were deposited in three zoned layers. The bottom layer, which corresponds to the top of the magma chamber, is relatively crystal-poor and consists of a nearly aphyric, highly differentiated phonolite. The top layer, which transported the contents of the bottom of the magma chamber with crystal enrichment, consists of a relatively crystalrich mafic phonolite. Hauyne is found throughout all three layers. In the Eifel district, the majority of the stones come from commercial pumice mine. Whenever a new layer is blasted from the high wall of pumice stone, collectors arrive to search for these rare blue crystals.

Powellite, CaMo₄, is one of the rarest gem materials. Transparent crystals suitable for faceting are known from India’s Maharashtra State. Indian powellite is colorless to yellow, and cut stones over 2 ct are very rare, although at least one attractive faceted stone over 50 ct exists.

Powellite from Chile has a completely different appearance. The locality is copper mine in the Atacama Desert. This Chilean powellite occurs as yellow pseudomorphs alter molybdenite, as small crystals and as very rare translucent green druses; the drossy material has yielded a few interesting faceted stones. The most transparent gemstones range from 0.5 ct to 1 ct, but translucent stones can weigh more than 15 ct. However, powellite’s low hardness (3.5-4 on the Mohs scale) makes it unsuitable for jewelry, despite this interesting color.

This contributor studied 18 faceted powellites to document their gemological properties. These samples were translucent to transparent. They ranged from yellow-green to green with weak pleochroism – in green and yellow-green. The refractive indices were over-the-limits of a standard refractometer, and the specific gravity values were rather high at 4.17-4.22. When examined with a polariscope, powellite behaves as an anisotropic aggregate; only the smallest cut stones show normal extinction. Very characteristic is a bright yellow fluorescence to short-wave UV radiation, but the material is inert to long-wave UV. The spectrum seen with a handheld spectroscope exhibited complete absorption in the blue and purple regions, starting from about 460 nm. When viewed with a microscope, the samples showed a swirled structure, although one stone exhibited a layered agate-like structure.

Because of its unusual color, the Chilean material is sometimes described as “chrome-powellite”. However, the color is probably due to trace amounts of copper. In addition to calcium and molybdenum, 0.02-0.30 wt. % CuO in yellow, green, and brown powellite, are reported with the greatest amounts measured in a dark green sample.
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Jeremejevite

Jeremejevite (Al₆B₅O₁₅[F,OH]₃) was discovered in the late 19^th century and named after Pavel V. Jeremejev, a Russian mineralogist and engineer. Most recorded faceted material is under 2 ct, but stones up to 5 ct are possible. Until relatively recently, the only two known localities for jeremejevite were at Mt. Soktuj, Transbaikal region, Russia and Cape Cross, Swakopmund, Namibia. Since the early 1980s, however, deposits have been reported from four localities in the Eifel volcanic area of Germany, where small or micro-crystals are found; from the southwestern Pamirs; and from the Fantaziya and Priyatnaya pegmatite veins of the eastern Pamirs, Tajikistan. Earlier 2001 year, an exciting new deposit was discovered in Namibia, about 180 km east of Cape Cross in the Erongo Mountains near Usakos.

The gemological literature contains very little information on jeremejevite, and most of the available publications list the color as being a blue similar to that of aquamarine. Colorless, pale blue-green, and pale yellow-brown jeremejevites are recorded too. Strunz and Wilk refer to the color of the material from Cape Cross as “cornflower” blue.

Therefore, we decided to investigate this unusually colored jeremejevite from Namibia and compare it to other yellow jeremejevites from Tajikistan and some blue jeremejevites. We also took the opportunity to perform advanced testing, to characterize the material further.

While faceted jeremejevite appears to have characteristic gemological properties (refractive indices with a lower limit of 1.640 and an upper limit of 1.651, birefringence constant at 0.009, and specific gravity ranging from 3.27 to 3.31), some of the data reported in standard gemological textbooks might cause confusion.

When gemstones, rare or otherwise, are set in jewelry or other objects, the examination restrictions placed on the gemologist can be formidable. The data gained by more sophisticated yet still nondestructive means can be particularly useful in such situations. The chemical data established the elements that should be present (major Al, and traces of Si, Mn, Fe, Cu, Zn, and Ga in the case of jeremejevite), the FTIR spectra – generated by instrumentation that is now available in major gemological laboratories – revealed areas of absorption that are common for this material, and the oriented Raman data readily identified those gem minerals as jeremejevite.

Главной особенностью англо-русского словаря является наличие в нем большого количества торговых названий ювелирных материалов. Эти названия часто встречаются в специальной геммологической литературе, в текстах реклам Интернета, на прилавках зарубежных ювелирных магазинов. При этом имеется большое число синонимов, особенно для синтетических аналогов природных минералов. Для составления англо-русского словаря был использован своеобразный англо-английский словарь P. G. Read «Dictionary of Gemmology» - с некоторыми исправлениями и дополнениями авторов.

Как в одном, так и в другом словаре нередко фигурируют через запятую несколько слов, соответствующих переводимому термину. Это не только синонимы, когда может быть использовано любое из приводимых слов. Чаще слова несколько различаются по значению, так что выбор того или другого определяется контекстом (конкретным текстом). Иногда правильный выбор возможен только при достаточном знакомстве переводчика с существом описываемых явлений или предметов. Тем не менее, студент не допустит большой ошибки, используя любое из приводимых через запятую родственных понятий.

В русско-английский словарь включены некоторые географические названия и многие названия минералов, в том числе и те, которые близки по транскрипции в русском и английском вариантах. Это сделано для того, чтобы не допускать мелких ошибок в написании английских эквивалентов, казалось бы, совершенно таких же русских слов.