GEM ARCHAEOLOGY
lyptic art, the craft of gem carving, dates at least to the the 7th millennium B.C., developing into a highly sophisticated industry in the Indus Valley Civilization from the mid-fourth to late second millennium B.C. As in other civilized societies, whether ancient or modern, gems engraved as sealstones and personal signets were not only ornamental adornments, but also were objects of indispensible utility, which, impressed into wax or clay, represented a legal signature, or personal identification, not unlike a driver's license or ID card today, or notarial imprimatur on business and bureaucratic transactions. The "device" or engraved part of a gem customarily was cut to read backwards in the stone, so that the engraving, such as a signature or right-facing portrait, would read right in the impression. An especially pretty or interesting-looking stone might be reserved for carving by only the best carver or only for the most important patrons. Pyrgoteles, for example, was the only gem cutter authorized to engrave the likeness of Alexander the Great in the rare gemstone, smaragdos (a green-colored stone from which the term for the modern beryl variety "emerald" is derived).
Art historians, archaeologists and connoisseurs have examined ancient texts—anonymously carved Mesopotamian inscriptions, Theophrastus's On Stones, Pliny the Elder's Natural History—as well as archaeological literature to inform our current understanding of the importance of gems and glyptic art in antiquity. From these texts scholars have inferred the gem varieties that were known in antiquity, their origins, and enhancement techniques that may have been applied to them intentionally by the ancient lapidary to improve or otherwise alter their appearance. The iconography or subject matter depicted in engraved gems, or intaglios, has been the primary focus of study.
Gemological and analytical study has been limited. Instead, the material attributes published on ancient gems have been reported anecdotally in successive journals, papers, and books, repeating for centuries erroneous interpretations of corrupt texts or observations based on sight alone, without the benefit of analytical verification.
Peri Lithon, or On Stones, written in the fourth century B.C. by Theophrastus, is the most important text about gems of the Classical world. It is the earliest scientific treatise on any subject extant in western literature, and is the antecedent of our modern scientific system of classification. Although it survives only as a fragmentary book, On Stones presents a taxonomy of the gems known in ancient times, describing their origins, physical properties, and magical or other special attributes. Gemological study offers an illuminating perspective on the ancient literature and a re-assessment of archaeological literature, enabling correction of some inaccurate information and clearer rendering of opaque passages, particularly concerning gemstone origins and the nature of accidental alterations effected through burial, for example, or treatments applied intentionally to gems by the ancient lapidary.
Various analytical techniques are used in gemstone identification, when standard gemological techniques prove inconclusive. A technique that has been employed for many decades is powder X-Ray Diffraction (powder XRD) analysis. Identification of an unknown sample is accomplished by comparing its crystalline structure against a reference base of known minerals.
Raman spectroscopy is being used increasingly to identify gemstones, employing a reference base of known gem spectra for comparison with unknown samples. Both qualitative techniques, such as X-Ray Fluorescence spectroscopy, and quantitative techniques, such as electron microprobe analysis, are useful in treatment detection and provenance studies, respectively