corundum, color.4, synthetic corundum.7, gems

The Heat Exchanger Method (HEM), a new crystal growth process, is in commercial production for 20 cm diameter sapphire crystals and 40 cm diameter silicon ingots for optical applications. The simplicity of the HEM, combined with a very high degree of control of the submerged, solid–liquid interface, allows growth of high-quality crystals. The HEM is also being adapted for the growth of Co:MgF₂, Ti:Al₂O₃, and Cr:Al₂O₃ crystals for laser applications.

corundum, USA.12, Montana.12, treatments.6, gems

change of color, alexandrite effect, gems

Gübelin discusses the alexandrite effect—the apparent change in color shown by certain gemstones under light sources of different spectral composition. Alexandrite chrysoberyl typically changes from blue-green or green in daylight to red or reddish violet in incandescent light, but comparable behavior also occurs in some garnets, sapphires, spinels, kyanite, fluorite and monazite. The phenomenon is explained in terms of absorption spectra having transmission regions in both the blue-green and red portions of the visible spectrum, separated by strong absorption in the yellow region. Which color predominates therefore depends on the spectral distribution of the illuminating light. The article relates the effect to the chromophoric trace elements present in the various gem minerals and emphasizes that the phenomenon is not restricted to chrysoberyl.

corundum, synthetic corundum.7, gems

Gübelin describes the diagnostic characteristics of the newer flux-grown synthetic rubies produced by Chatham, Kashan and Knischka. Particular emphasis is placed on microscopic examination of their characteristic inclusions, some of which may resemble features encountered in natural ruby and therefore complicate identification. The article illustrates and compares the internal features of the three products, showing how inclusion morphology and growth-related characteristics can be used to distinguish these synthetic rubies from natural stones. The paper contains 42 figures, 41 in color.

gems, Islamic art, Islamic metalwork, Islamic jewelry, Islamic gems, ruby, diamond, spinel, emerald, jade

This was the first outline of Islamic Iranian metalwork to be published in any language. It is based on the collection in the Victoria & Albert Museum and is the work of Dr. Assadullah Souren Melikian-Chirvani [b. 1936], the distinguished authority of Near Eastern metalwork. The many illustrations present relevant details of epigraphy and iconography, the great majority of which have not been previously published. Souren Melikian is a French-Iranian art historian, art critic, and curator. He is a renowned scholar of Iranian culture, and its sphere of influence.

corundum, geology.11, gems

corundum, color and luminescence.4, gems

The absorption spectra of vanadium-containing solids, especially of oxides and silicates, are described; new spectroscopical data are given for V³⁺ in axinite, diopside, tremolite, muscovite, roscoelite, cyanite, kornerupine, and tourmaline. The spectra consist of two strong absorption bands in the visible area, which are assigned to the ligand field transitions ³T₁(F) → ³T₂(F) [ν₁] and ³T₁(F) → ³T₁(P) [ν₂] of V³⁺ in octahedral or distorted octahedral oxygen coordination polyhedrons. The absorption maxima of V³⁺ are observed between 18,500 and 15,100 cm⁻¹ (ν₁) as well as between 26,700 and 22,500 cm⁻¹ (ν₂). From the positions of both absorption maxima, the values of the ligand field parameters Dq (from 1990 to 1660 cm⁻¹) and B (from 640 to 510 cm⁻¹) are calculated for the different crystal structures. The absorption spectra of V³⁺ are compared with the data of chromium-containing substances with similar or identical host lattices. In some silicates and oxides, an isomorphic replacement of Mg²⁺ and/or Al³⁺ by V³⁺ or Cr³⁺ is possible only in one distinct lattice site or in several sites of the host lattice, showing only small differences in their M–O distances: alum, GASH, spinel, corundum, garnet, topaz, axinite, beryl, pyroxene, amphibole, mica. The spectroscopic data of these solids are compared with the absorption spectra of minerals with two or more octahedral lattice sites with different M–O distances. In the tourmaline lattice, a preference of the smaller Al-site compared with the greater Mg-site by V³⁺ is deduced. The absorption spectra of zoisite are explained by a superposition of the bands of V³⁺ on the smaller Al1,2-site and the bands of the oxovanadium complex VO²⁺. The lattice position of V³⁺ in cyanite and kornerupine could not be deduced from spectroscopical data. The ligand-field parameter Dq, as well as the position of the maxima of ν₁ and ν₂ in vanadium- and chromium-containing solids, are dependent upon the average metal–oxygen distance r of the VO₆ or CrO₆ polyhedron of the host lattice and of the cation polyhedron of the oxygen anions. An increasing M–O distance r due to a lattice expansion causes a shift of ν₁ and ν₂ to smaller wavelengths; the ligand field parameter (Dq) is also reduced (distance effect). An increase of the average M–O distance, i.e., a lattice expansion, is possible to be caused by an increase of the concentration of the transition metal cation (here V³⁺ or Cr³⁺, concentration effect), by an isomorphic replacement of other cations in solid solution series (diadochic exchange effect), or by vacant lattice sites in solid solution series (loosening effect in structures). Another reason for a shift of ν₁ and ν₂ to smaller wavelengths, as well as a decrease of Dq, is the contra-polarisation of the oxygen anions caused by an interaction with neighbouring cations (contra-polarisation effect). In general, this effect is smaller in oxides and larger in silicates; it can be approximately estimated for V³⁺ and Cr³⁺ in different lattice types: chlorite.

synthetic corundum.7, flux, gems

corundum, Tanzania.12, Kenya.12, gems

corundum, synthetic corundum.7, gems

Gübelin discusses the identification of newly available synthetic and treated blue sapphires, concentrating on Chatham flux-grown synthetic sapphire and on natural sapphires whose color has been modified by heat treatment or surface diffusion. He describes the crystal habits and physical and optical properties of rough and faceted Chatham material, followed by a detailed examination of diagnostic inclusions, some of which closely resemble inclusions in natural sapphire. Twenty-three color photomicrographs illustrate these features. A further 15 photomicrographs document characteristics of color-enhanced sapphires, including altered mineral inclusions surrounded by tension cracks, pockmarked girdle surfaces, pronounced color zoning, and concentrations of color in surface-reaching fractures.