corundum, quality.10, gems

Corundum “type” categories are presented as a practical system for describing the overall color appearance of rubies and blue sapphires, teaching colored stone grading and communicating international market preferences. Developed from a classification introduced by Henry Ho in 1981, the system requires consideration of the combined effects of hue, intensity, tone, fluorescence, pleochroism, inclusions, color zoning and cutting proportions. Types describe appearance rather than geographic origin and are not intended as a ranking of relative value. The author argues that traditional geographic descriptions, such as “Burmese” ruby or “Ceylon” sapphire, can identify recognizable appearances shared by stones from several sources; consequently, origin alone should not determine a stone’s category or justify a quality premium. Definitions distinguish hue position and intensity, assessed through flashes of brilliance, from tone, assessed through the overall lightness or darkness of the stone. Windowing, extinction, pleochroism and zoning receive particular attention because they alter the perceived distribution and strength of color. Five ruby categories are described: Type A, predominantly pure red; Type B, pinkish red with lower color intensity; Type C, slightly orangy red, commonly with inclusions that scatter light and reduce extinction; Type D, blackish red with high tone, strong extinction and weak fluorescence; and Type E, fluorescent purplish pink to red, commonly with silk and comparatively little visible extinction. Four blue sapphire categories are distinguished: Type A, intensely colored, slightly violetish “royal blue”; Type B, “milky blue,” with minute particles or silk that diffuse light; Type C, brighter, lower-toned “cornflower blue,” often with visible zoning; and Type D, dark “inky blue,” commonly associated with iron absorption, color banding and conspicuous greenish-blue pleochroism. The categories are then applied to purchasing preferences reported for Japan, France, Germany, England, Italy, Switzerland, the United States, Australia and the Middle East. These descriptions encompass differences in preferred color, clarity, proportions and finish, while recognizing that purchasing patterns also reflect affordability and resale conditions. Japanese and German buyers are described as favoring comparatively lighter appearances, English and Australian buyers as commonly accepting darker material, and French and Italian buyers as placing greater emphasis on color than on clarity or proportions. Despite these market differences, the author reports broad agreement among connoisseurs on exceptional quality: ruby combining intense fluorescent red with a slight yellow component, and sapphire combining intense, slightly violetish blue with good clarity, moderate tone and a bright appearance. A slight milkiness producing a velvety texture is also prized by some sapphire connoisseurs. Type classification is ultimately recommended as an aid to understanding the interacting causes of color appearance and matching stones to particular markets.