Advances in geophysical technology and computer imaging now permit mapping of geologic features at various depths beneath the Earth’s surface in more detail than ever before. Geophysical methods,which for some years have been used to explore for diamonds (and many other minerals),also hold promise as important tools in both exploration for new colored stone deposits and assessment of known deposits. Particularly exciting are the use of seismic-reflection and radar-imaging techniques for high-resolution mapping of near-surface pegmatites,veins,metamorphic layering in crystalline rocks,and alluvial deposits in sedimentary rocks. These techniques can produce images of regions only a few centimeters to a few meters in size down to several meters to tens of meters below the surface. Thus,it is now possible to locate directly some buried structures containing gem deposits.

Much of the exploration for gemstones (even diamonds,in some regions) is conducted with relatively simple,often primitive,techniques. In many areas of known gem occurrences,development and mining involve little more than shovels and screens for alluvial deposits,and explosives plus mechanical tools (such as pneumatic drills) for hard-rock deposits (e.g.,pegmatites; figures 1 and 2). In areas where gems are not known to occur,most discoveries are made by chance. This contrasts with the exploration procedures routinely used for many other Earth resources,such as oil and gas or metallic minerals,in which years of regional geologic and geochemicalmapping are usually followed by intensive periods of acquiring and analyzing geophysical data. Once target areas are located,more detailed— and usually more expensive—exploration methods (such as drilling or excavation) can be undertaken. However,geophysical techniques have seldom been used to delineate gemstone,particularly colored stone,deposits.

Figure 1. The discovery of pegmatites,and of gembearing pockets in a pegmatite dike,is one of the greates challenges in gem exploration. These bicolored tourmalines from the Himalayamine,San Diego County,California,represent some of the fine gems waiting to be discovered in pegmatites worldwide. Courtesy of Pala International,Fallbrook,California; photo © Harold&Erica Van Pelt

Figure 2. This pegmatite pocket from California’s Himalayamine yielded thousands of carats of fine tourmaline. Unfortunately,the discovery of such pockets typically requires hundreds of meters of tunneling and a great deal of luck. Rapid improvements in geophysical prospecting techniques offer the hope that this situation will change in the near future. Photo © Harold&Erica Van Pelt.