The Study On Gemological Characteristic Of Red To Yellow Series Garnets

Garnet is one of the medium-quality gemstones which has long and rich history and diverse designing styles. It gains popularity throughout the world because of its varies colors and highly performance price ratio.Recently, in jewelry market, many businessmen classify garnets in subspecies names rely solely on color and famous origins, and this causes the disorder understandings and confusion in pricing. In terms of gemstones identification, the vast majority of authentication institution canno t name the subspecies of garnet, and this causes consumers cannot have the satisfied identify answers.This study is based on this problem which doesn’t have a fast and nondestructive method to identify garnets in subspecies names, and this study select red to yellow series garnets which are common seen in the jewelry market. Using conventional gemology testing methods, UV-vis spectroscopy, electron microprobe, Raman spectroscopy, Infrared spectroscopy and other modern means of testing methods, author try to summarize the gemology characteristic of garnets systematically, and tend to discuss identifying garnets in subspecies names using nondestructive, quick and accurate method.Electron microprobe analysis and triangular diagram of garnets show that purple garnets, rose-red garnets and claret-red garnets are almandine, yellowish-red and orange-yellow garnets are grossular, brighter yellowish-red and orange-yellow garnets(fanta garnets) are spessartine.Raman spectra of red and yellow series of garnets show specific characteristic Raman frequencies. The diagnostic Raman shifts of almandine are about 912cm- 1、552cm- 1、355cm- 1, spessartine are about 900cm- 1、543cm- 1、341cm- 1, grossular are about 874cm- 1、543cm- 1、368cm- 1. When testing unknown samples with Rama n spectra, comparing to these end-members Raman shifts, can nomenclature garnets in subspecies names. This result is consistent with microprobe analysis. And we can preliminarily judge that, Raman test can be used as an effective ways of naming garnets.Infrared spectra of red and yellow series of garnets show specific characteristic absorption. We can determine garnets in the “identify” area which is 400-1500cm-1. The characteristic absorption of garnets are three strong and sharp peaks appeared in 800-1000 cm-1, and three less sharper peaks appeared in 400-700cm-1.UV-vis spectroscopy shows that Fe3+ and Cr3+ cause the color of red series garnets, when having higher content of Fe3+, the color is much deeper and darker; when having higher content of Cr3+, the color is much lighter and brighter. Fe3+ cause the color of yellowish-red and orange-yellow garnets, and Ca2+ makes no contribution to the cause of color. Mn2+ cause the color of brighter yellowish-red and orange-yellow garnets(fanta garnets), and Cr3+ cause the red tone of color.