gems, general gemology, precious stones

An introductory manual of gem identification for students and practising jewellers, this American revision of Robert Webster’s Practical Gemmology combines elementary scientific explanation with descriptions of testing instruments and concise reference accounts of gem materials. Virginia V. Hinton’s revisions adapt the work to American practice, adding information on equipment, terminology and commercial standards. The opening chapters introduce the qualities underlying gem value and distinguish minerals, rocks, crystals, crystalline aggregates and amorphous substances. Elementary crystallography explains crystal systems, symmetry, axes and twinning, relating internal structure to optical behaviour, identification and cutting. Physical properties include cleavage, fracture, hardness and lustre, followed by detailed treatment of specific gravity and its determination through hydrostatic weighing, heavy liquids and other methods. The optical chapters explain reflection, refraction, total internal reflection, dispersion, polarisation and double refraction before describing refractometers and alternative methods of measuring refractive index. Colour is treated as an aid requiring confirmation by instrumental testing, with discussions of pleochroism, dichroscopes, absorption spectra, colour filters, fluorescence and phosphorescence. A chapter on microscopy covers magnification, illumination and the examination of features useful in identifying gems and their substitutes. The principal gem species are then summarised through chemical composition, crystallography, diagnostic constants, appearance, varieties and geographical occurrence. Expanded diamond coverage discusses inclusions and surface blemishes, cutting proportions, the meaning of flawless, and the importance of standard magnification and controlled lighting in clarity and colour assessment. Subsequent chapters distinguish synthetic gems from imitations, describe their production and recognition, and examine glass, plastics, assembled stones and artificially induced colour. Cutting styles are explained in relation to the appearance and optical properties of fashioned stones. Organic materials receive separate treatment, especially pearls, their formation, cultivation, imitations and methods of distinguishing cultured from natural examples. Coral, amber, jet, tortoiseshell and ivory are also described, followed by unusual gem species and ornamental, decorative and curio stones. Diagrams, photographs, tables of constants, chapter recapitulations, test questions and answers support instruction and reference use. The emphasis throughout is on accessible principles and practical discrimination, with more intricate scientific tests deliberately left to advanced works.