This report is an updated version of an Open File report (Walton, 1996) describing the geology and geochemistry of gemstone deposits and applicable exploration criteria.The 1996 report, and this update, are both aimed at exploration geologists and prospectors who are unfamiliar with the geology of gemstone deposits and how to explore for them. Its objective is to provide information on gemstone deposit geology and provide general guidelines for the identification of, and exploration for, gemstone occurrences. The report focuses on a selection of some of the most valuable gemstones in the global gem trade: ruby, sapphire, emerald, chrysoberyl, tsavorite garnet, tanzanite and the gem minerals (topaz, aquamarine, tourmaline) found in permatites or rhyolites. Exploration criteria for other gemstones such as opal, nephrite jade, jadeite jade, zircon, lapis lazuli and rhondonite are beyond the scope of this report. For each gem mineral covered in this report, background information is introduced and the physical and chemical characteristics of the gem are presented. The geology and genesis of each gem deposit type is summarized and exploration guidelines are listed. One of the difficulties in developing exploration guidelines is that many of the world's gem deposits are unique. It is difficult to determine the range of variations and physico-chemical conditions under which a particular type of gemstone deposit may form, if there is only one deposit to study.