and intermediates. Surprising is the large number of transmitters (type II). In the 0.8-mm group, 16 percent are completely transparent. Partially transparent intermediates comprise 57 percent of the total; complete absorbers, 27 percent. Handlers of diamonds know that type II occurs more often among smaller than among larger diamonds, but ,the true frequency of type II is known only very vaguely if at all: "maybe one in a thousand" or "possibly one in a hundred." Such guesses usually refer to

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... rystals exceeding 0.2 carat, while our main group of diamonds averaged 0.0015 carat. Our observations establish that in our size range about 1/6th of the diamonds are completely ultraviolet-transparent, and that about half are partially transparent--transparent in local patches. Therefore we suggest that many diamonds begin life as ultraviolet-transparent type-II crystals; as they grow, most begin to incorporate ultraviolet-absorbent regions. Since our crystals were all visually transparent and clear, it is logical to postulate that these absorbent regions are diamond of type-I character 'and not merely opaque foreign inclusions-witness the significant change in distribution shown by our group of large crystals. The 16 percent completely transparent among our smaller crystals falls to 8 percent for our larger crystals. The intermediates constitute 41 percent, but ,the type-I diamonds (completely ultraviolet-opaque) have changed from 27 percent among the smaller crystals to 51 percent for the larger. We do not think that this marked increase in absorption is due to a simple logarithmic increase (akin to Lambert's law) caused by increase in thickness; rather we attribute it to the probability during growth of incorporation of thin yet strongly absorbent layers. Clearly, if only a few sufficiently absorbent layers of type-I material are incorporated during growth, the whole crystal can easily become quite opaque, despite the fact that the absorbent layer may account for only a fraction of the total thickness. On this hypothesis of mixture one can predict that the number of transparent crystals will fall rapidly as their size increases, so that we soon arrive at the guess estimatesless than 1 percent-usually made for and intermediates. Surprising is the large number of transmitters (type II). In the 0.8-mm group, 16 percent are completely transparent. Partially transparent intermediates comprise 57 percent of the total; complete absorbers, 27 percent. Handlers of diamonds know that type II occurs more often among smaller than among larger diamonds, but ,the true frequency of type II is known only very vaguely if at all: "maybe one in a thousand" or "possibly one in a hundred." Such guesses usually refer to crystals exceeding 0.2 carat, while our main group of diamonds averaged 0.0015 carat. Our observations establish that in our size range about 1/6th of the diamonds are completely ultraviolet-transparent, and that about half are partially transparent--transparent in local patches. Therefore we suggest that many diamonds begin life as ultraviolet-transparent type-II crystals; as they grow, most begin to incorporate ultraviolet-absorbent regions. Since our crystals were all visually transparent and clear, it is logical to postulate that these absorbent regions are diamond of type-I character 'and not merely opaque foreign inclusions-witness the significant change in distribution shown by our group of large crystals. The 16 percent completely transparent among our smaller crystals falls to 8 percent for our larger crystals. The intermediates constitute 41 percent, but ,the type-I diamonds (completely ultraviolet-opaque) have changed from 27 percent among the smaller crystals to 51 percent for the larger. We do not think that this marked increase in absorption is due to a simple logarithmic increase (akin to Lambert's law) caused by increase in thickness; rather we attribute it to the probability during growth of incorporation of thin yet strongly absorbent layers. Clearly, if only a few sufficiently absorbent layers of type-I material are incorporated during growth, the whole crystal can easily become quite opaque, despite the fact that the absorbent layer may account for only a fraction of the total thickness. On this hypothesis of mixture one can predict that the number of transparent crystals will fall rapidly as their size increases, so that we soon arrive at the guess estimatesless than 1 percent-usually made for the larger crystals. If correct, our interpretation directly opposes the usual conception regarding the scarcity of type-II diamond. We