Notes on Gems, U. Texas Gem Class

Preface Opal, rubellite, citrine, sapphire, aqua., tourmaline & ruby

   These notes are my attempt to compile some of the available information on the most popular gems, those that are covered in lectures during the second half of the course. Most of the material comes from the reference list. The notes for most gems are far from complete; there are, for example, whole books on diamonds, beryls, tourmalines, garnets, etc.. I have summarized those aspects that I either found interesting or that I felt were, at a minimum, some basic facts one ought to have on hand. I strongly encourage those of you who are interested in learning more (and there is a lot more to learn!) to consult the references or peruse the excellent collection of materials in the Walter Geology Library on the 3rd floor of the Geology Building. A somewhat dated but still good starting point is Gill's Index, which is in the Geology Library reference collection. An updated version is now online.

    Each of the gem summaries is divided into several sections. After a brief introduction that defines precisely what the gem is and the gemological names that are used to describe it, the physical and optical properties useful for identification are given. In compiling these data, I relied heavily on the book by Anderson, and to a lesser extent the recent book by O'Donaghue. These books list data that are sometimes at odds with other sources, but I have endeavored to thoroughly check the data presented here and in instances where there are disagreements give ranges of values. Listed ranges are probably representative of true variations in properties that result from minor variations in chemical makeup. The most widely cited sources that gives similar information are the books by Liddicoat and Webster.

    Following the physical and optical properties is a short summary of properties that are most useful for distinguishing the gem from similar appearing gems, synthetics, or imitations.

    A section on occurrence and sources summarizes the geologic setting of historically and currently important sources for the gem, as well as any distinctive features unique to gems from particular localities. Until very recently, information on these topics was extremely meager, widely scattered among a variety of sources, and in many instances contradictory. The situation has been remedied somewhat with the recent publication of Keller's excellent books, which give good accounts of the geological setting and source of several of the most important gems. Many of the issues of the quarterly journal Gems and Gemology (also in the Geology Library) also contain good articles on several of the important gem localities (see Gill's Index).    The shaping and treatment section considers those aspect of the gem that require special attention during cutting, and some of the common treatments used to enhance color or appearance. The latter is a very interesting, somewhat complex subject that is admirably covered in the book on gemstone enhancement by Nassau. I have attempted to summarize some of the salient treatments, but much of this information is incomplete. An information guide on this topic, based upon the AGTA's Gemstone Enhancement Manual, is available online.

    Finally, the section on pricing and valuation describes aspects that influence price and attempts to give rough (sometimes very rough) estimates of wholesale prices. Several things need to be considered when looking at these. First, I had an extremely difficult time finding information on current gems prices (such information is generally not available in the public domain) and came to rely on informal surveys of dealers I conducted at the 1989 and 1991 Tucson Gem and Mineral Shows, and information from catalogues and friends who are in the gem trade. More recently, price trends and current prices for popular gems have become available via the web. In selecting price source links, I have largely ignored commercial gem and jewelry web sites (of which there are now a vast number) and focused instead on industry organizations or other noncommercial sources that track wholesale prices.

   Whereas there is generally good agreement on the attributes that define the best gem of a given species, these are not easily quantified (especially for colored stones), resulting in a wide range of prices for similar-appearing goods. For color stones, perhaps 50-60% of their value resides in the quality of the color, with the remainder being influenced by clarity (20-30%) and cut (10-20%). In general, top-grade colored stones will have pure hues, vivid tones, and an even, strong color saturation. Likewise clarity for such stones at 10X magnification will be exceptional, but this is, of course, also a function of the gem material (e.g. emerald vs. aquamarine). You should have a good understanding of cut by this point in the class. It is worth noting that while we have and will continue to emphasize cut, the colored stone industry by and large still does not consider cut in the same regard. It has been my experience that well-cut, commercial grade or better colored stones are a rarity; the relatively small importance given to cut in pricing probably reflects the low standard most dealer have come to accept as the norm. (The same is not true for diamonds however, where a poor or mediocre cut can greatly influence price - see the notes).

    The best I was able to do with limited information was define fairly broad ranges of prices one might expect to pay, and even these are subject to question. Given these considerations and the high volatility of some gem prices, the prices given should be viewed as, at best, a very general guide. Though I do not discuss controls on market volatility or factors influencing trends in gem prices in any detail, a highly readable source of such information is the book by Marcum, which also contains some prices and general advise on gems as investments. An online source is Robert Genis' National Gemstone web pages.

    Second, and equally important, please keep in mind that these are wholesale prices for unmounted gems. Retail prices can be 1.5 to 2.5 times more expensive. Retail jewelry settings may also have a markup in this range, and jewelry prices usually include an additional cost for mounting the gem(s).

   I hope you will find these notes useful and that they will continue to serve as a reference after the course is finished. If in reading and studying them you can learn a fraction of what I did by writing them, they will have served their purpose. Good Luck!

Mark Helper

©1998
Updated 09/13/17
Comments and questions to helper@mail.utexas.edu
Department of Geological Sciences
The University of Texas at Austin