topaz, gems

D.B. Hoover’s Topaz (1992), published in the Butterworth-Heinemann Gem Books series, is a comprehensive mineralogical, geological, historical, and gemological study of topaz. The author examines the mineral from its earliest historical references to modern scientific investigations, emphasizing the relationship between its chemical composition, crystallographic structure, physical properties, geological formation, and use as a gemstone. The opening chapters investigate the etymology of the name topaz and the historical confusion surrounding its identification, particularly the application of the name to other yellow and green gemstones. Famous topaz crystals and faceted gemstones are described, followed by a systematic examination of the mineral’s chemistry, crystal structure, and physical and optical properties. Hoover discusses variations in refractive index, specific gravity, and other measurable characteristics, questioning whether some properties are more closely associated with geological origin and conditions of formation than with colour alone. He also examines evidence suggesting that certain topaz specimens may possess triclinic rather than strictly orthorhombic symmetry, challenging conventional assumptions about the mineral’s crystallography. The treatment of colour encompasses natural colour variations, luminescence, and the processes responsible for colour changes, including artificial colour modification through irradiation and heating. Crystal morphology receives detailed attention, together with the characteristics and significance of inclusions, which provide information relevant to both gem identification and the geological environment of crystallization. The geological sections describe the formation of topaz in different rock types and mineral deposits, emphasizing the conditions under which fluorine-bearing fluids and magmatic processes contribute to its development. A substantial geographical survey examines important occurrences around the world, documenting the geological settings, associated minerals, and gem-producing potential of individual localities. Particular attention is devoted to the celebrated topaz deposits of the Ouro Preto district of Minas Gerais, Brazil, including their geology, history of mining, and production of the orange, pink, and reddish varieties associated with imperial topaz. The volume also considers the scientific and commercial significance of large crystals and exceptional faceted examples, placing them within the broader history of gemstone collecting and the gem trade. A particularly important theme running throughout the book is the substitution of hydroxyl (OH) for fluorine in the topaz structure, which Hoover shows can produce previously unrecognized variations in crystallographic, physical, and optical properties. The work is illustrated with colour photographs and black-and-white figures and is supported by an extensive bibliography. In a 1993 review in The Journal of Gemmology, Michael O’Donoghue praised the clarity of Hoover’s writing, the strength of his scientific evidence, and the originality of his mineralogical interpretations, recommending the book as an essential reference for gemologists.