corundum, synthetic corundum.7, Chatham, flux, gems

Groups of Chatham synthetic ruby and blue sapphire crystals are examined to identify features potentially useful for recognizing faceted stones subsequently cut from this material. The specimens comprise four groups of each variety, with individual crystals displaying a tabular or platy habit. Blue colour in the sapphires is unevenly distributed, commonly concentrated towards crystal tips, whereas the rubies appear strongly carmine red when viewed through sufficient depth and pale pinkish red through thinner sections. Surface frosting, bubble-bearing junctions and residual coatings are described but considered unlikely to survive cutting as useful identifying features. Internal characteristics offer greater promise: both varieties contain abundant dark angular platelets, possibly metallic, together with two-phase inclusions, twisted or veil-like feathers and angular growth or colour zoning. The platelets are especially noteworthy for their profusion, although comparable forms can occur in natural ruby. Sapphire spectra show an easily observed absorption band at 450 nm in deeper blue areas, accompanied by much weaker bands at 460 and 471 nm. Thus, unlike conventional Verneuil synthetic blue sapphire, the material can display the familiar sapphire absorption complex. Refractive indices measured on one sapphire surface are normal for corundum, while the ruby absorption spectrum is also conventional. Luminescence provides more distinctive evidence. The synthetic sapphires show patchy yellow fluorescence under long-wave ultraviolet radiation, identified as their most useful diagnostic feature, whereas the rubies exhibit strong red fluorescence and pronounced, persistent phosphorescence after x-ray excitation. Ruby phosphorescence lasted approximately 2–20 minutes following exposures of 10 seconds to 10 minutes. X-rays also induced temporary green or yellow colours in colourless and pale blue sapphire regions; these faded under the heat and light of a tungsten bulb, while deeper blue areas remained unchanged. Isolated colour changes following ultraviolet exposure could not be reproduced consistently. No faceted examples had been examined at the time of writing, so the conclusions remain predictions based on crystal specimens. The author expects combined inclusion and luminescence observations to identify most future faceted material, while recognizing possible difficulties with unusually clean stones and emphasizing caution because x-ray testing can alter sapphire colour.