jade, jadeite, fei cui, nephrite, Wyoming, Burma, inlusions, gems

ekanite, gems

Gübelin gives a detailed gemological account of ekanite, a newly recognized metamict gem mineral from Ceylon, named for F. L. D. Ekanayake, who first recognized its unusual character. The material occurs in the gem gravels near Ellawala, northeast of Ratnapura, and is typically greenish-brown to yellowish-green, transparent to translucent, and sometimes displays chatoyancy or asterism caused by oriented clouds of minute inclusions. Gübelin describes its gemological properties, including its singly refractive character, refractive index of about 1.595, specific gravity of about 3.28, and weak absorption bands, together with microscopic features and inclusions. Chemical investigation showed calcium, thorium, silicon, aluminium, and lead, while measurements demonstrated significant radioactivity associated particularly with thorium. The destruction of the original crystal lattice through radioactive decay accounts for its metamict condition. Gübelin also discusses its probable original crystalline state, chemical composition, and age as estimated by thorium-lead dating, establishing ekanite as an unusual new addition to the gem minerals of Ceylon.

corundum, synthetic corundum.7, gems

Describes ruby specimens consisting largely of natural cores covered by thin synthetic layers, reportedly grown hydrothermally. Their physical constants match natural ruby, and inclusions within the cores can make identification misleading. Minute gas bubbles in the coating and a sharp boundary between core and overgrowth reveal their composite nature; the observed features suggest growth from a melt rather than a hydrothermal process.

corundum, Brazil.12, gems

Describes sapphire crystals from the Coxim River area of Mato Grosso, Brazil, noting unusual rounded inclusions resembling the gas bubbles commonly associated with synthetic corundum.

phase-contrast microscopy, gems

Explains the application of Zernike’s phase contrast microscopy to reveal subtle internal features in gemstones. By converting differences in optical path into visible brightness contrasts, the method enhances thin, transparent inclusions and structures whose refractive indices closely resemble those of their hosts. Specially adapted equipment permits examination within gemstones, revealing exsolution, alteration and healed fractures that are difficult to distinguish under ordinary illumination.

corundum, USA.12, Idaho.12, gems

A corundum occurrence examined in August 1950 in Valley County, Idaho, approximately five miles east of Smiths Ferry, provides an example of the mineral preserved in its original rock matrix and a possible model for the source of corundum recovered from Idaho gold placers. Earlier reports recorded corundum and sapphires from numerous placer localities, including tributaries of the Payette River, creeks near Pierce, the Stanley Basin, Salmon River gravels and the Burgdorf district, but their source rocks had generally remained unidentified. A historical account from Rock Flat near Meadows suggested derivation from a weathered basaltic dike, although this interpretation had not established an unequivocal occurrence in original matrix. At the newly described locality, corundum occurs as metacrysts within schist inclusions enclosed by granodiorite of the Idaho batholith. The largest inclusion observed measured approximately thirty feet across, and the limited dimensions were considered to preclude commercial development. The corundum-bearing schist consists principally of dark olive-green biotite, oligoclase of approximately An11 composition and corundum, the latter constituting an estimated 15–20% of the rock. Corundum crystals reach three centimetres in length and two centimetres in diameter, although crystals only one to two millimetres long are also common. Biotite grains average two to three millimetres across, while the much finer feldspar averages approximately 0.1 millimetres and tends to concentrate around the corundum. Minor associated minerals include an unidentified amphibole, chlorite, muscovite, magnetite and martite; muscovite, magnetite and martite also occur as inclusions within corundum. Well-formed corundum crystals are absent: many grains are irregular, some are nearly tabular, and most display a linear orientation within the schist’s foliation plane. Whole crystals range from light grey to buff, whereas crushed fragments are predominantly clear, with subordinate very pale blue, lavender and pink material. Selected material yielded a pycnometer specific gravity of 3.91. Refractive indices were not accurately determined, but identification was confirmed by an X-ray pattern. Although the locality itself was not regarded as a potential commercial source, the occurrence suggests that comparable metamorphic rocks could supply placer corundum. Examination of the narrow metamorphic belt bordering the Idaho batholith approximately twelve miles farther west is therefore proposed as a worthwhile direction for further investigation.

diamond, formation, geology, gems

Sinkankas: A truly monumental work of increasing scientific importance, not so much for its information on diamond and its mining as for its detailed studies of the petrology and mineralogy of kimberlitic pipe-fillings. Such treatment of originally deep-seated mantle-derived rocks was well ahead of its time in providing clues as to the nature of the mantle and the formation of the crust in general. The significance of Williams’ studies escaped even himself, and certainly most others, as evidenced in the niggardly review accorded the work by E.H. Kraus, American Mineralogist, 18, 4, 1933, 176, who saw it as merely “supplementing and bringing up-to-date the monumental and classic work of his father.” Because of the greatly increased interest in mantle problems and plate tectonics, the work now appears to be one of those inadvertently fortunate amassments of accurate data which find applications at a later time. The lukewarm reception accorded the book resulted in the issue of relatively few copies, and the book is now difficult to find. It is far from a “popular” presentation, such as might be said about his father’s general work on the diamond mines of South Africa, despite the remark of Kraus noted above, and is most useful to the petrologist and geologist rather than to the gemologist, although it does contain a great deal of interest to the latter as well. Volume I opens with a short chapter on modern mining practice at Kimberley and then describes the geological relationships of the intrusive kimberlites to host rocks and possible origins, with details on the kimberlites themselves. Practically all of this volume is devoted to petrography. The second volume continues similar studies with emphasis upon the inclusions in kimberlites, but now with much space devoted to the diamond itself, taking up such matters as the characteristics of diamonds from various mines, speculations as to their genesis, inclusions in diamond, crystallography and X-ray analyses, physical properties, chemical composition, and growth phenomena. The final chapter treats diamondiferous alluvial deposits of South Africa and the diamonds found therein. There is a long table of diamonds that weighed over 100 carats on pp. 619–636; it is mostly a listing of crystals from the Kimberley district, although some large Brazilian stones are included in an appended list. As may be expected from a largely petrological work, many of the color plates depict kimberlite thin sections with careful descriptions of the minerals therein, but other plates are of more general interest, one showing the now-famous collection of colored diamond crystals formed by the author during his tenure in De Beers. From the gemologist’s standpoint, the work is deemed most important for its studies on inclusions in diamond. Extended review by A.T.J. Dollar, The Gemmologist, 2, 16, 1932, 111–115; 18, 1933, 181–183; 19, 205–207; 21, 268–272; 22, 305–310.

corundum, ruby, sapphire, Sri Lanka.12, gems

A concise account of the crystalline rocks of Ceylon, intended to summarize their character and classification as then understood. Coomaraswamy discusses the terminology of “gneiss” and “granulite,” preferring the latter for many Ceylonese rocks, and regards them as belonging to the Charnockite Series described from southern India, the two regions forming a closely related petrographical province. He describes the principal rock types and their mineral banding, together with the crystalline limestones and rocks intermediate between limestone and granulite. The latter may contain diopside, phlogopite, amphibole, scapolite, spinel, sphene, calcite and, more rarely, serendibite, and may occur at limestone–granulite contacts, as inclusions, dykes or bands; some also yield mica of commercial value. Coomaraswamy argues that, whatever their ultimate origin, the crystalline limestones were essentially contemporaneous with the granulites and crystallized under broadly comparable high-temperature conditions. He also distinguishes the wollastonite-bearing Galle group and discusses the structural geology of the crystalline series, particularly foliation, folding and the influence of rock structure on the development of parallel strike valleys such as those around Hatton. The paper is an important early synthesis of Ceylon’s Precambrian crystalline geology and provides geological context for the mineral and gem deposits associated with these rocks.

gems, gemology, lapidary

Sinkankas: An extremely rare work on gemstones that emphasizes practical aspects of lapidary treatment and other matters that would be of value to the tradesman and jewelry maker. It is further remarkable for including a chapter on the methods by which Indian lacquerware is manufactured, which of course has nothing to do with gems. Gemma discusses the origin of gemstones in the first chapter, considering at length Robert Boyle’s statements on the same as given in his Exercitatio de Origine et Viribus Gemmarum, supporting some and disputing others. Other citations and quotations from authorities appear throughout the text. Other topics treated include the magical and curative powers of gemstones, physical properties, color and its enhancement by use of foils, the pricing of gems, the manufacture of doublets, artificially colored and imitation gems, and substitutes. One chapter is devoted to “figures” in stones, that is, clouds or swarms of inclusions, banding, and other markings which resemble human or other figures, shells in fossiliferous limestones, insects in amber, and stars in star stones. All of these are treated soberly as natural manifestations and are not thought to be magical or miraculous in nature as had been done not too long before Gemma’s time. Another chapter treats petrification due to mineralized waters. From the practical viewpoint, this work is especially rich in details on the cutting of gemstones and the equipment required to do so, including the art of gem engraving. Gem-cutting equipment then in use is described in minute detail, as well as a process for classifying raw tripoli to make it useful as a polishing agent. In all respects this admirable, reasoned work deserves greater recognition.