COBBING AND TESTING
When time permits, the field buyer turns his attention to the pile of “doubtful” material mentioned above. This contains crystals and frag¬ments of potential gem material which needs to be separated from waste. A typical example may be a large quartz crystal in which smoky quartz alternates with citrine, with the latter of course being much more valu¬able than the former. The crystal may also contain flawed areas which must be separated from the clean areas, and usually the base is milky quartz of no value whatsoever.

Such a crystal is turned over to an ex¬pert “cobber,” or a workman skilled in the use of small whippy hammers which he utilizes delicately to remove unsatisfactory material as shown in Figure 2. Very large sections may be separated by the use of presses or diamond saws. Despite the loss of weight necessarily incurred in the cobbing process, the remaining material is upgraded to such an extent that it brings a price which more than makes up for the weight loss and cost of labor.

Quartzes and other transparent gemstones are also tested by immer¬sion methods, a technique which will be elaborated upon in a succeeding chapter. In brief, the crystals are immersed in an oil which enables the expert to see inside and detect the presence of flaws. This is important not only because of gem-cutting considerations but also because much quartz is sold for the making of electronic devices and needs to be free of flaws and other defects if it is to be useful. Much of our quartz gem-stone production from Brazil is sold at reasonable cost only because it is a by-product of mining principally directed toward the recovery of electronic-grade quartz crystals.

HEAT TREATMENT
Another important test applied to certain rough gemstones is heat treatment to determine if colors can be improved thereby. Heat treat¬ment is accomplished by the field buyer upon such gemstones as citrines containing smoky overtones (the smoky color is driven off), amethysts changed to rich brownish-red or green), yellow topaz (changed to pink) and certain greenish aquamarines (changed to blue). He may test a few specimens out of each lot and then certify that the color change will occur, or he may heat-treat all specimens and charge more for the final product accordingly.