corundum, synthetic corundum.7, gems

Gübelin and Knischka describe a new gem-quality synthetic ruby produced by Paul O. Knischka using a proprietary “gradient technique” involving supersaturation and controlled cooling. The resulting crystals have an unusual nearly isometric or pseudocubic habit and exceptionally numerous well-developed, undamaged crystal faces, with examples possessing 32 or 38 faces. The authors compare several types of Knischka synthetic ruby with natural and Verneuil synthetic ruby, examining crystal morphology, trace-element chemistry, optical and gemological properties, fluorescence, and inclusions. In color and general appearance the material closely resembles natural ruby, although it tends toward a more violet red; refractive index, birefringence, and other standard properties offer little assistance in separation. Microscopic features include folded or feather-like inclusions, color zoning, two-phase inclusions, negative crystals, and occasional metallic platelets, some of which can resemble features in natural stones. The study emphasizes the increasing difficulty of distinguishing high-quality synthetic ruby from natural material and the potential diagnostic value of inclusions and trace-element composition.