Figure 1. In this energy level diagram of trace elements in corundum, trivalent donor ions are shown on the left and divalent acceptor ions are shown on the right. A charge-compensating donor-acceptor pair can form when the donor level is significantly above the acceptor level. There is no significance to the lateral positions of the energy levels, but the vertical positions are very significant. Data from Kröger (1984b).
Figure 2. As displayed by these rough and cut sapphires, Potentate’s sapphire production includes a wide variety of colors. Photo by Eric Welch/GIA, courtesy of Potentate Mining.
Figure 1. This 12.61 ct blue sapphire and 7.90 ct orange sapphire recently mined by Potentate show that large material still comes out of Rock Creek, Montana. Photo by Eric Welch/GIA, courtesy of Potentate Mining.
This negative crystal, measuring 1 mm in length, contains a large graphite crystal and carbon dioxide. When cooled below the supercritical point, the CO2 exists as a gas and liquid; gas bubble gaseous CO2 is visible due to the difference in refractive index between the liquid and gas phases (B–F). When heated above the supercritical point, the CO2 homogenizes into a supercritical fluid (A and G). The flat surface of the negative crystal gives rise to a high nucleation density of gas bubbles, which ultimately coalesce into a single large (B and E). Photomicrographs by Nathan Renfro.
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