spinel, Tanzania, Umba, gems

corundum, ruby, Vietnam, gems

corundum, Tanzania.12, gems

Describes rubies, blue sapphires and other colored corundums from Tanzania, with particular attention to their gemological characteristics. The authors characterize the colors quantitatively using a chromaticity-coordinate diagram and examine the stones’ absorption spectra and mineral inclusions. The study compares the different color varieties and relates their optical behavior to their composition, providing analytical data useful for characterizing Tanzanian gem corundum. A contemporary Journal of Gemmology abstract specifically notes the treatment of color, inclusions and absorption spectra.

gems, beryl, emerald, Colombia

Coffee-table style photo essay on the emeralds from Colombia, with some lovely photography of the mines at Muzo, markets, trading and cutting in Bogota. The last few pages offer a bit of gemological information about emerald and where it is found around the world. This book is valuable mainly for the photos.

gems, spinel, color-change spinel, Sri Lanka, cobalt spinel

A laboratory examination documents an unusual colour change in a 2.24-carat natural cobalt-coloured spinel mounted in a man’s ring. The stone appeared dark, richly saturated, slightly violetish blue under fluorescent illumination and dark purple under incandescent light. Its contemporary setting, combining white and yellow metals, enhanced the visual effect. Testing at the West Coast laboratory established properties consistent with natural cobalt-blue spinels examined previously, including examples reportedly originating in Sri Lanka, although the note does not establish a geographic source for this particular specimen. A Duplex II refractometer yielded a single refractive-index reading of 1.720. Polariscope examination showed a singly refractive response accompanied by very slight anomalous double refraction. The stone displayed weak red transmission and appeared moderately red through a Chelsea colour filter. Examination with a handheld spectroscope revealed the diagnostic absorption band of natural cobalt spinel at approximately 460 nm, together with additional bands near 480, 552, 559, 575, and 622 nm. Broad general absorption extended from approximately 660 nm towards longer wavelengths. These optical observations supported comparison with the laboratory’s earlier experience of natural cobalt-blue spinel despite the specimen’s unusual change between blue and purple. Microscopic examination beneath one end of the crown revealed a cluster of moderately sized inclusions comprising a well-formed octahedral crystal and several whitish crystals of irregular shape. Rain-like strings of pinpoint inclusions radiated from this group and extended through the stone, accompanied by numerous small, fine needles. The included crystals and needles are described by their appearance and morphology without assignment to specific mineral species. The report combines observations under different light sources with refractometry, polariscope testing, colour-filter examination, absorption spectroscopy, and microscopy to characterize the mounted gemstone. Its significance lies in documenting a distinctive blue-to-purple colour change in a stone whose other measured properties and internal features were consistent with natural cobalt-blue spinel.

corundum, synthetic corundum.7, gems

gems, archaeology, archaeogemology, India, Sri Lanka, Pakistan

Beads from four sites involved in Early Islamic trade (7th to 12th century) are representative of the role the Muslim world played in the Indian Ocean bead trade. The continuation of Classical techniques, the Islamic trade’s self-sufficiency, and the insight beads provide concerning past behavior are some of the issues explored.

gems, topaz, rutilated topaz, treated topaz, heated topaz

Colorless faceted topaz containing acicular inclusions has been marketed as “rutilated” topaz, though earlier studies revealed that these inclusions are actually limonite-stained etched dislocation channels rather than rutile. This article discusses the application of heat treatment to convert the limonite staining into hematite, thereby enhancing the visibility and altering the color of the inclusions to make them appear more prominent.

gems, history of gemology, history of mineralogy, Arabic mineralogy, history of science, Egypt, Rome, Greek science, astronomy, ancient reference

Written some time before 100 BC the On the Erythraean Sea of Agatharchides of Cnidus is the most important source for an almost forgotten chapter of the history of geographical discovery, the exploration of the Red Sea and the region surrounding it by agents of the Ptolemaic government of Egypt in the century after the death of Alexander the Great in 323 BC. It also contains the earliest extensive account of the geography and ethnography of the coasts of northeast Africa and western Arabia. Although the original text of the On the Erythraean Sea is no longer extant, three abridgements of unequal extent and quality by the first century BC historian Diodorus, the first century AD geographer Strabo, and the ninth century AD Patriarch of Constantinople Photius survive. The present edition contains the first English translation of all three epitomes of this important work together with an introduction and extensive notes analysing the historical background and significance of Agatharchides' book.

gems, spinel, Tanzania, spinel inclusions

A brief gemological report on spinels from Tanzania’s Umba Valley, occurring in pink, red, orangish red and bluish violet colors. The specimens examined had refractive indices of 1.713–1.718 and specific gravities of 3.54–3.69. Spectroscopic examination was used to investigate the causes of the different colors. The note documents the range of gem spinel then reaching the market from the Umba Valley and provides basic properties useful for characterizing this East African material.