There is no specific ore associated with gems rather there is a whole suite of rock types termed an environment containing one or more minerals that are classified as a gem. These rocks range from sand and gravel deposits to ultramafic igneous rocks that are composed of dark colored minerals. Even if you identify a specific rock that contains a specific gem the odds are still about 300 to one it will develop into a commercial deposit. The best way to prospect for gems is to choose a specific gem and learn everything about that gem you can. More gem deposits have been found in the library through diligent research then going into the field unprepared.
Once you have chosen a specific gem you will then have to find out everything you can about that specific gem. This begins in a library where you do your basic research on that specific gem. You find out what it is composed of, in what environment, and finally pay close attention to any pictures that show the surrounding rocks. This is only the first part of a very complicated and long process, but it is a good beginning. You can also find out what kinds of gems are in your area at your local historical society by reading your town’s history. You are not usually the first person that has gone looking for gems in your area. Even more information is available from the US Geological Survey (USGS) or your state geologist.
The next step in this process is a trip to a major museum that has a rock and mineral collection. While at the museum study all the gems they have on display especially those that interest you. After you have made a thorough study of the gems you then go to the rock collection to look at the specific rocks your favorite gem is found. Study everything you can about these rocks while you are in the museum so you can identify them when you finally get out into the field. The trip to the museum will afford you a chance to see the rocks in specimens large enough so that when you finally take to the field you will readily recognize the rocks that are apt to contain gems, and those that are barren.
When in the field you will be able to put the lessons you have learned in both the library and the museum into practice. When you see a rock outcrop you should be able to make positive field identification. Even this isn’t the end because samples of the rock have to be taken into the lab for a positive identification using various instruments.
Like any science, geology or prospecting begins in the classroom or library. The better you are forearmed with knowledge the better you can perform your job. The most important book you should study is a fieldbook on rocks and minerals. As your abilities become more advanced there are other books that you can read especially those that deal with mineralogy and petrology. These are also some very good periodicals available that deal specifically with gem hunting that give many tips and hints about finding gems. Most of these can be found on the Internet by entering the subject matter into a search engine.
Geology of Gem Deposits, ed. Lee A. Groat, Springer Verlag, 2008, http://www.springerlink.com/content/x3808h2267r56w54/
Smithsonian Gem & Mineral Collection, Ronald Louis Bonewitz, 2005