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.