opal, Mexico, gems

Gübelin provides a historical and technical survey of Mexican opal, discussing the principal varieties and their major localities. He notes the use of opal in pre-Columbian Mexico and describes the important deposits in the volcanic regions of Querétaro and Jalisco, where opal occurs in cavities and fractures in rhyolitic rocks. Particular attention is given to Mexican fire opal, with its characteristic yellow, orange and red body colors, as well as transparent precious opal displaying play-of-color. The article discusses the appearance, occurrence and characteristic inclusions of Mexican material; later studies cite Gübelin for two- and three-phase inclusions and mineral inclusions including hornblende, goethite, hematite, fluorite, quartz, cristobalite and chalcedony in Querétaro opal.