corundum, synthetic corundum.7, gems

Webster surveys synthetic crystalline materials developed since 1946 that have entered the gemstone trade or offer potential for ornamental use, bringing together scattered literature and results from his own examinations. Arranged alphabetically by chemical or species name, the review includes synthetic equivalents of natural minerals and manufactured compounds without known natural counterparts. Established pre-war Verneuil corundum and spinel receive only limited attention, while crystal-growth methods are outlined rather than discussed in detail. Identification is approached through physical and optical constants, internal features, absorption spectra, fluorescence and phosphorescence, X-ray transparency, electrical conductivity and, where relevant, magnetism. Particular attention is given to synthetic emeralds from Chatham, Gilson, Zerfass and Linde, together with Lechleitner’s synthetic-emerald-coated beryl and layered constructions. Earlier synthetic emeralds commonly differed from natural stones in refractive indices, birefringence and density and showed characteristic veil-like inclusions and strong luminescence. More recent products increasingly overlap natural emeralds in their measurable properties; the addition of iron can suppress luminescence, making careful examination of inclusions especially important. Flux-grown and hydrothermal rubies likewise present greater identification difficulties than conventional flame-fusion material, with seed remnants, feathers, distinctive inclusion patterns and radiation responses providing possible evidence of synthesis. The diamond section records the progression from commercially useful industrial crystals to General Electric’s May 1970 announcement of gem-quality synthetic diamonds of approximately one carat or more, while noting that production costs still exceeded those of comparable mined stones. Extensive coverage is also given to garnet-structured compounds, particularly yttrium aluminium garnet, and to diamond simulants including strontium titanate, rutile and lithium niobate. Their usefulness and detection are considered in relation to dispersion, birefringence, density, hardness, colour and durability. Other entries discuss coloured synthetic quartz, fluorite, scheelite, periclase, silicon carbide, zincite, zircon and several less familiar or experimental materials. Developments in spinel include red flame-fusion stones, flux-grown crystal groups used uncut in jewellery, sintered material imitating lapis lazuli and material displaying a moonstone effect. Throughout, trace-element additions are shown to influence colour, spectra and luminescence, so that specimens of the same nominal material may behave differently. Photomicrographs, spectral diagrams, a comparative radiograph, tables of constants and a selected bibliography support the descriptions. The compilation distinguishes established commercial products from experimental or merely prospective gemstones and provides a contemporary reference for the increasingly varied identification problems posed by synthetic crystals.