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.