UNITED STATES MINERAL RESOURCES 



GEM STONES 



By Robert E. Thaden 



CONTENTS 



Abstract of conclusions 247 



Introduction 247 



Exploitation 248 



Geologic environment 248 



United States occurrences 249 



United States production 250 



Imports and exports 250 



Selected references 250 



ABSTRACT OF CONXLUSIONS 



Gem stones are mineral substances, other than metals, 

 that are attractive enough for use in personal adornment or 

 as small household objects. More than 230 mineral species 

 are, or have been, used as gem stones at one or another 

 stage of man's cultural history. Gem stones ideally possess 

 beauty, durability, and rarity, but the value of a stone may 

 be enhanced considerably by masterful cutting. Diamond, 

 sapphire, ruby, and emerald generally are considered pre- 

 cious stones; all other gem stones generally are considered 

 semi-precious. Neither imitation nor synthetic gems have 

 infringed seriously on the market for natural stones, nor 

 are they likely to do so. Gem stones occur in most of the 

 major geologic environments, but they do not form ore de- 

 posits in the normal sense. Since 1935, the mining of gem 

 stones in the United States has been almost entirely a rec- 

 reational activity of mineral collectors and hobbyists. The 

 annual value of such gem production has risen from half a 

 million dollars in 1952 to nearly $3 million in 1972. A little 

 over $500 million a year is spent for importing gems, mostly 

 cut and uncut diamonds, and about $200 million worth of 

 cut diamonds and synthetic gems are exported annually. 



INTRODUCTION 



Gem stones are minei'al substances, other than 

 metals, that are attractive enough for use in personal 

 adornment or as small household objects such as 

 book ends, pin boxes, and statuettes. Gem stones are 

 mostly, but not exclusively, of inorganic origin. 

 Some vegetable substances, such as amber, and the 

 cannel, anthracite, and jet varieties of coal are in- 



cluded, as well as some organically precipitated 

 mineral materials, especially pearl and coral. 



A large number of minerals have been used as gem 

 stones at one or another stage of man's cultural his- 

 tory, mostly depending on availability or fashion. 

 An inventory of more than 230 mineral species that 

 now have or have had currency as gems — nearly one- 

 tenth of all known minerals (Fleischer, 1971) — can 

 easily be compiled from published lists of gems and 

 gem localities (Ball, 1931; Jahns, 1960; Schlegel, 

 1957; French, 1968). 



Ideally a gem stone should possess beauty, dura- 

 bility, and rarity. Beauty, of course, is in the eye of 

 the beholder and varies with the individual and with 

 the culture in which he lives. Beauty is based on an 

 appreciation of certain attributes of the stone — its 

 color, clarity, optical dispersion, index of refraction, 

 and perfection of chatoyant zones, to name a few. 

 Durability encompasses other physical properties in- 

 cluding chiefly resistance to scratching, which is a 

 measure of innate hardness; resistance to cleavage 

 and fracture; and insolubility. Rarity appeals to 

 human exclusiveness and perhaps for this reason 

 usually is the principal control over the value as- 

 signed to stones. Rarity, however, can act in the 

 opposite manner; Jahns (1960) has stated that some 

 stones, even those having all the other desirable at- 

 tributes, are so rare as to be undependable in supply, 

 to be virtually unknown, and therefore to command 

 no market. Examples given by Jahns include the 

 beautiful oil-green and reddish-brown strongly 

 dichroic andalusite, and transparent phenakite, 

 euclase, and staurolite. Others also could be named — 

 transparent cassiterite, brazilianite, and lapis lazuli. 

 Some stones are a matter of national pride and are 

 far more expensive in their country of origin than 

 elsewhere. Alexandrite (chrysoberyl) from the 

 U.S.S.R. is the prime example. 



Another characteristic, independent of the physi- 

 cal properties important in determining the value of 



U.S. GEOL. SURVEY PROF. PAPER 820 



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