During the study of the individual gemstones later in this course, all of the sources of each stone will be mentioned briefly. The purpose of this assignment is to show the relative importance both of different types of gem deposits and of the producing countries.
In contrast to the diamond industry, for which accurate figures are published regularly for almost every producing country, figures for colored stone production are almost nonexistent. Those few that are available are far from reliable. The reasons for this are related to the average size of the mining operation in each of the important producing areas, plus the absence of a central buying and marketing organization such as that in the diamond industry. There are almost no colored stone mining operations anywhere in the world that compare with even an average size diamond mine. The tendency of the usual small operation is to conceal information, on both production and sales figures. In addition, in many foreign countries there is a strong tendency to conceal production from government taxing authorities, with the result that total production can only be estimated. Most of the colored stone mining areas are marginal and produce only when demand is high enough to yield a worthwhile price. Few colored stones could be produced, even under ideal conditions, in quantities that would make exploitation comparable to that of diamond feasible. Although the most important colored stones are in sufficient demand to keep mining a constant activity in the major districts, even the most important districts in the world contain these gemstones in a concentrations too low to warrant large scale exploitation in a high cost country such as the United States.
By value, there is no question but that the most important sources
of both colored stones and diamonds are the secondary,
or alluvial deposits. Regardless of the mode of formation
of the important gemstones, those that are tough withstand the rigors
of transportation by streams from the original deposit and are concentrated
in alluvial deposits. Some primary deposits furnish too low a yield
of gem-quality material per ton of rock mined or it is too difficult
to mine, to permit commercially profitable mining, whereas nearby alluvial
deposits may contain sufficient concentrations of gem rough to be operated
to advantage of the primary types of colored stone deposits, the most
important is the pegmatite dike, followed in order by contact metamorphic
deposits and vein deposits.