corundum, inclusions.5, gems

synthetic corundum.7, synthetic ruby, Geneva ruby, gems

Anderson discusses early synthetic rubies commonly called reconstructed rubies, combining observations from the London Chamber of Commerce Gem Testing Laboratory and his personal collection with historical accounts of their manufacture and sale. He uses pre-Verneuil as a convenient chronological designation for material predating commercial operation of Verneuil’s furnace, while acknowledging Verneuil’s earlier experimental activity and the possible circulation of his technical knowledge. Such stones occasionally reached the laboratory in jewellery and could initially resemble natural rubies because their colour was less garish than that of later synthetics and they contained numerous impurities. Lens or microscope examination nevertheless revealed their synthetic character. A photomicrograph obtained in November 1958 records one of nine early synthetic rubies mounted in an antique silver snuffbox bearing a Georgian hallmark dating from 1800; the setting’s age was inconsistent with the stones. Anderson draws attention to a prominent flask-shaped bubble whose apparent direction of movement is opposite to that usually seen in Verneuil material, suggesting that the original boule might have grown as a stalactite rather than a stalagmite. He presents this interpretation cautiously and recommends closer study of bubble shapes and distributions. Another specimen is a small boule grown around a visible fragment of natural Siamese ruby. Although formerly regarded as reconstructed material, Anderson accepts the possibility that it was produced by a flame-fusion technique resembling the later commercial Verneuil process, with the natural fragment serving as support and seed. Five additional early synthetic rubies in a gold ring show a rhodolite-like colour, dark rounded and fragmentary inclusions, minute bubbles, strongly marked and tightly curved striae, pronounced dichroism, and complete extinction between crossed polarizers. Historical discussion centres on accounts of the jeweller Ohlat or Oblatt, who reportedly manufactured rubies by fusing small natural fragments. Anderson recounts a story involving financial backing, pawnbroking, and an alleged fraud trial, while acknowledging uncertainty over details and calling for archival verification. A separate published account describes oxyhydrogen burners, a platinum–iridium tube, and a closely guarded cooling procedure. These narratives are treated as potentially informative but less secure than scientific records. The article also revisits the distinction between natural and laboratory products, rejecting the implication that using natural ruby as feedstock makes the resulting synthetic equivalent to a natural gemstone. Anderson concludes that early predictions of synthetic stones destroying the natural-gem market proved unfounded, while emphasizing the continuing historical and gemmological interest of these sparsely documented materials.

corundum, inclusions.5, gems

Set of color plates in a folder by John Koivula displaying various inclusions in gemstones. Although his name is not on it, this is the first stand-alone publication of John Koivula’s photomicrographs on gems. Due to the manner in which it was bound, copies are extremely scarce today.

corundum, Burma.12, gems

Describes the Mogok ruby-producing region of Burma (Myanmar), outlining the occurrence and mining of its rubies. A colour geological map and other illustrations document the area, while colour photomicrographs show characteristic inclusions in the gemstones.

gems, corundum, inclusions.5, beryl, emerald, garnet

The new edition of this well-known text follows familiar lines. The first 160 pages are occupied by a glossary including cross-references to later sections of the book and details of synonyms, incorrect names and constants. Following this section is an account of the crystal systems and other physical and chemical data. The section on absorption spectra is expanded and improved in this edition with clear diagrams of spectra pertaining to the newer synthetics. As may be expected in a book by these authors, the chapters on inclusions are comprehensive. Although illustrations in the text would have been useful here (and generally throughout the work), there is a booklet of black-and-white photographs of inclusions in a pocket at the end of the book. A chapter on synthetics includes such unlikely candidates as BaF2 and MgF2, the first of which is slightly water-soluble; but there is no mention here or anywhere in the book of gadolinium gallium garnet, now on the market as a cut diamond simulant, nor of Gilson synthetic opal. This section is, however, very well written and comprehensive. A reading of Linde for Gilson on p. 340 is one of the rare misprints. For a book of this size and scope the coloured illustrations are not worthy and the world map of gemstone deposits is too crowded. A more serious criticism is the presence of material in the body of the book which appears neither in the glossary section nor in the short index. However, the usefulness of the work is considerable. Reviewed by Michael O’Donoghue

corundum, ruby, sapphire, Umba.12, Tanzania.12, East Africa.12, inclusions.12, gems

gems, corundum, inclusions.5

Recently, a paper was published describing inclusions of corundum within corundum. The stones in question are light blue sapphires with a tinge of green, or alternatively brownish or purple-colored corundums. They are found in the Umba mine near the Umba River in northeastern Tanzania, close to Mount Kilimanjaro. The inclusions are elongated needles that, at first glance, resemble the well-known rutile needles often seen in corundum. Figure 1 illustrates that the needles follow three directions that appear to be at right angles to one another. Figure 2 reinforces this impression; however, in reality, the angles differ slightly but measurably from 90°. According to the cited publication, these needles run at right angles to the faces (42̅1), (22̅1), and (24̅1), which are steep hexagonal pyramid faces. X-ray analysis revealed that the needles are composed of corundum and occur in “twin positions” relative to the host crystal. Surprisingly, each of these needles is themselves twinned parallel to the prism faces of the surrounding corundum. Furthermore, the needles are of secondary origin, having formed after the host crystal. This, along with the fact that the inclusions induce considerable internal stress, suggests that the needles may have formed due to pressure. However, this theory has not yet been conclusively proven.

corundum, ruby, Thailand.12, Cambodia.12, inclusions.5, gems

Reports electron-microprobe identifications of characteristic inclusions in Thai rubies, relating their microscopic appearance to mineral composition and considering their possible geological significance. Gübelin describes mineral grains associated with flat, approximately circular halos or reticulated films of fluid inclusions, commonly aligned with twin or parting planes. These features had been observed in many thousands of specimens but could not be identified reliably by conventional gemmological tests. Rubies containing unusually large inclusions were therefore selected in Bangkok and polished until the enclosed grains were exposed for analysis. Yellowish crystals containing calcium and phosphorus were identified as apatite; brown to reddish-brown grains containing iron, aluminium and silicon were assigned to almandine garnet. Black, metallic inclusions consisted principally of iron and sulfur and were identified as pyrrhotite. One composite inclusion contained nickel-bearing material interpreted as minute pentlandite grains within pyrrhotite, surrounded by a chalcopyrite envelope. Element-distribution images illustrate these relationships. Separate colourless, rounded inclusions were identified as plagioclase, while one slightly greenish crystal surrounded by tension fractures was identified as diopside. Many other transparent features proved to be negative crystals, whose original gas or liquid contents were lost during polishing. The author stresses that small rounded inclusions may resemble gas bubbles and therefore require careful examination when distinguishing natural ruby from synthetic material. Competing explanations for the associated fluid halos include fracture formation followed by healing and primary incorporation of foreign fluid during interrupted crystal growth. Gübelin favours a primary origin involving disturbances in growth, supersaturation and the enclosure of residual fluid, while acknowledging the difficulty of reconstructing the process. He interprets the recurring inclusion assemblages as evidence of similar formation conditions among rubies recovered from scattered Thai alluvial deposits. Their sulfides and associated silicates, together with the reported absence of calcite, spinel and rutile silk, lead him to suggest interactions between crystalline schists or granitic pegmatites and basaltic rocks as a possible source environment. The paper establishes analytical support for several inclusion identifications while presenting the proposed growth and geological models as interpretations.

corundum, Burma.12, Sri Lanka.12, Thailand.12, gems

Sinkankas: In this pictorial popular work, the author provides a nontechnical description of the major gemstones in a language that stresses the beauty and fascination of gems and deliberately suppresses dry scientific data. Thus the book begins with several chapters extolling the wonders of gemstones, their colors and origins, the special beauty of gems that display color or reflective phenomena, inclusions in such gems as moss agates, and a little on the splendid crystals from which gems are cut and their sources in nature. Other chapters touch on the arts of the lapidary, the diamond, the corundum family, and many other gemstones, ending with a chapter on the Idar-Oberstein agate-cutting and carving industry, and closing with a very brief chapter on the true value of gemstones in their aesthetic appeal. The book includes a glossary in which technical terms are defined and the basic properties of gemstones mentioned elsewhere are summarized. There is no index nor bibliography. The most striking feature is the large number of superb color photographs, ranging from faceted gems, some obviously selected from the finest available for illustration, to cabochons, carvings, natural crystals, views of lapidary work in progress, scenes in famous gem-mining areas, jewelry, and objects of art made from stone. These are of particular value and reflect the author’s wide travels. Not seen. Reviews: Zeitschrift der Deutschen Gemmologischen Gesellschaft, 18(4), 1969, pp. 193–194; Der Aufschluss, 21(12), 1970, p. 384.