corundum, synthetic corundum.7, treatments.6, gems

CIA’s faceting instructor, Bill Kerr, had in his possession a boule of German-produced synthetic corundum. After rounding the larger end and polishing it, he noticed how the three rays crossed each other to form the expected six-ray star and then extended down the sides of the rough boule still attached. This inspired the attempt to cut a cat’s-eye stone oriented 90° to the c-axis of the boule, resulting in the 6.23 ct synthetic cat’s-eye ruby shown on the left in figure 12. On the right of this double-exposure photograph is the same stone viewed on the end and parallel to the c-axis—one-half of the star is now visible. Another section of the same boule (toward the bottom) that was later worked into a stone contained part of the transparent seed crystal with a shallow overgrowth of the asteriated material. This material, when cut, produced a weak star with an open center caused by the non-asteriated seed-crystal section—a very unusual stone indeed (see figure 13).