GEM STONES 



249 



scattered sparsely throughout a large body of rock, 

 or have crystallized as small aggregates or fill veins 

 and small cavities. Even placer concentrations tend 

 to be small — a fevi? stones in each of several bedrock 

 cracks, potholes, or gravel lenses in a streambed. The 

 average grade of the richest diamond pipes in Africa 

 is about 1 part diamond in 40 million parts "ore," 

 and because much diamond is not of gem quality, the 

 average stone in a woman's engagement ring is the 

 product of the removal and processing of 200-400 

 million times its volume of rock. 



Gem stones occur in most of the major geologic 

 environments. Each environment produces a char- 

 acteristic suite of gem materials, although many 

 gems occur in more than one environment. 



The igneous rocks are the source of the greatest 

 value of gems each year largely because the kimber- 

 lites in Africa (see chapter on "Abrasives") yield 

 most of the world's diamonds. Other mafic igneous 

 rocks yield ruby, sapphire, garnet, and peridot, and 

 probably also, at least in some places, jadeite and 

 nephrite. Granitic igneous rocks, particularly the 

 coarsely crystalline pods and tabular bodies called 

 pegmatites, and coarsely crystalline fillings of miaro- 

 litic cavities, probably contain a wider variety of 

 gem stones than any other rock type. Quartz, tour- 

 maline, topaz, apatite, spodumene, and beryl (includ- 

 ing emerald), in all their varieties and colors, as 

 well as zoisite, garnet, chrysoberyl, sphene, spinel, 

 and zircon, are characteristic of pegmatites and ig- 

 neous cavity fillings. 



Metamorphic rocks are the source of ruby, saph- 

 phire, garnet, and emerald. The schists and gneisses 

 also contain andalusite, actinolite, chrysoberyl, zois- 

 ite, topaz, jadeite, and nephrite, whereas the addi- 

 tional suite of gem stones characterizing the marbles 

 and related calcareous metamorphic rocks includes 

 rhodonite, lazurite, epidote, cordierite, apatite, axin- 

 ite, and tourmaline. Contact-metamorphosed rocks 

 are likely to be more productive of a variety of gem 

 materials than those that have been regionally meta- 

 morphosed; the igneous rock bodies nearby supply 

 fluid emanations containing the constituents needed 

 to form some of the gem minerals. 



Aqueous solutions, both the hot solutions derived 

 from plutonic igneous bodies or from volcanic ac- 

 tivity and the relatively cold solutions that percolate 

 downward into the rocks from the surface of the 

 earth, are the source of a rich variety of gem ma- 

 terials which precipitate in cracks, fissures, and 

 voids or replace preexisting materials. Some gems 

 precipitate only from waters that are at least rea- 

 sonably hot; examples are euclase, citrine quartz, 

 imperial topaz, emerald, and benitoite. Other gems 



can precipitate from aqueous solutions having a wide 

 ranges of temperature — cacoxinite and amethyst, 

 for example, as well as opal, pumpellyite, datolite, 

 thomsonite and the other zeolites, hematite, tourma- 

 line, some topaz, and certain garnets. Cold-water 

 solutions descending through the rocks are the 

 source of most agate, jasper, petrified wood and 

 bone, and tigereye and other forms of cryptocrystal- 

 line quartz, and they also supply the oxygen needed 

 to alter preexisting mineral materials to such gem 

 materials as malachite and turquoise. 



When a primary deposit of gem stones is weath- 

 ered deeply, as is common in many parts of the 

 world, the gems are left behind in a clayey matrix 

 from which miners can easily wash them. Nearly all 

 mining of gem-bearing rock is confined to the weath- 

 ered zone because of the difficulty of freeing gems 

 from fresh rock without shattering them. 



A placer deposit is formed when wind or water 

 removes clayey matrix from a weathered gem-bear- 

 ing rock body leaving the gems concentrated. Placers 

 generally are far richer than the original sources 

 because of the concentration process and because the 

 imperfect gem stones tend to be eliminated by break- 

 age, leaving behind the gems of exceptional quality. 



UNITED STATES OCCURRENCES 



Nearly 100 different mineral species have been 

 mined and marketed in the United States as gem 

 stones, and every state has contributed to this mar- 

 ket. Most deposits consist of "semiprecious" stones. 

 Even quasi-gem materials such as banded rhyolite, 

 porphyritic andesite, clusters of marcasite crystals, 

 and fossil trilobites have commercial value in the 

 fabrication of pins, broaches, tie slides, and similar 

 items. The United States also has produced diamond 

 from Arkansas and ruby and sapphire from Mon- 

 tana and the Appalachian States, especially North 

 Carolina; North Carolina also continues to produce 

 an occasional emerald. Maine was at one time known 

 for its commercial production of colored tourmalines 

 and beryl. Large quantities of pink tourmaline from 

 the Pala district of California were sent to China in 

 the early part of this century. Wyoming and Cali- 

 fornia continue to produce large quantities of the 

 jade minerals, but production from vast jade fields 

 in the Kobuk River region of Alaska is just begin- 

 ning. Benitoite is known to occur nowhere in the 

 world except in California, and the rhodolite gar- 

 nets of North Carolina and the pumpellyite amyg- 

 dules from the western end of Lake Superior are, as 

 far as is known, unique. 



Large deposits of the more valuable gem materials 

 that can be and have been mined for centuries such 



