corundum, Sri Lanka.12, inclusions.5, gems

Schmetzer describes unusual three-phase inclusions in an untreated yellow sapphire from Sri Lanka. The inclusions occur as tabular negative crystals oriented with their broad surfaces parallel to the basal plane of the host corundum. Each contains a transparent, doubly refractive solid crystal together with liquid and gas phases. At the time of this study the phases could not be identified conclusively; Schmetzer suggested that the solid might be mica, while the fluid and gaseous components were probably CO₂. Subsequent work by Schmetzer and Medenbach (1988) resolved the question. Examination of more than 300 Sri Lankan sapphires showed that comparable three-phase inclusions contain liquid and gaseous CO₂ plus needle-like to tabular crystals of diaspore, the latter identified by X-ray diffraction.

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

The blue oval cut shown in figure 11, which was submitted to the New York lab for identification, is interesting for a number of reasons. The stone has an extremely deep pavilion and a shallow crown, with a total depth-to-width ratio of approximately 115%—so that it is actually deeper than it is wide. In addition, to the unaided eye it appears to be free of inclusions and has a pleasant light blue color. These facts suggested that the stone might be an old, “native cut,” unheated Ceylon sapphire. However, careful examination with the microscope revealed a shadow of curved color banding near the culet and suspicious stress cracks on some facets. The strong chalky blue fluorescence to short-wave UV radiation suggests that the stone either was subjected to extreme heat in treatment or is of synthetic origin. The curved color banding together with the Plato test positively identified it as a Verneuil flame-fusion synthetic sapphire. The stone is also an excellent example of how a very small color zone can produce a pleasant face-up color. Notice in the side view (figure 12) that the top of the stone is colorless; the small blue area near the culet, visible at this angle, is virtually the entire extent of color. Blue synthetic sapphire boules often have color only in a thin layer near the surface. Figure 12 also shows the extreme depth-to-width ratio of this stone.

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

opal, inclusions, gems

corundum, cat’s eye star sapphire, Burma.12, phenomena, inclusions.5, gems

Schmetzer and Kiefert examine an unusual chatoyant sapphire from Burma in order to determine the cause of its cat’s-eye effect. Microscopic examination shows numerous fine, elongated inclusions arranged in a strongly preferred crystallographic orientation. The authors study their orientation in relation to the corundum crystal structure and show that reflection and scattering from these oriented inclusions produce the sharply defined eye. The stone’s internal growth structures, inclusions, optical properties and absorption characteristics are also described and compared with those of other sapphires. The study illustrates how crystallographically oriented inclusions can produce chatoyancy in corundum and documents a relatively uncommon phenomenal variety of Burmese sapphire.

corundum, ruby, sapphire, Nepal.12, inclusions.5, gems

Kiefert and Schmetzer describe pink and violet sapphires from a newly recognized occurrence in Nepal. Associated minerals include calcite, phlogopite, margarite and rutile. The pink color is attributed principally to chromium, whereas the violet coloration results from a combination of Cr, Fe and Ti. The corundums commonly contain twin lamellae and dense sets of straight, parallel growth planes that separate zones differing in color and concentrations of fine mineral particles. Particularly characteristic are small, doubly refractive mica platelets, identified as margarite, some occurring as the solid component of three-phase inclusions. Numerous healed and unhealed fractures are also present and substantially reduce the gem quality of much of the material.

corundum, ruby, Nepal.12, inclusions.5, geology.11, gems

corundum, sapphire, inclusions.5, Malawi.12, gems

corundum, inclusions.6, synthetic corundum.7, Burma.12, gems, Kashan, Ramaura, Linde, beryl, emerald, synthetic emerald, Raman spectroscopy

By allowing positive identification of fluid and solid inclusions inside minerals, laser Raman microspectrometry has proved to be a perfect technique for discriminating between natural gems or synthetic minerals made for jewellery (sapphire, ruby, emerald)

corundum, inclusions.5, gems