This text describes the theory and practice of optical mineralogy in terms useful to all practitioners from the beginning student to the professional in field and laboratory geology and industrial and environmental mineralogy. The author's aim is to provide the simplest possible access to the most powerful techniques of optical crystal identification. The book emphasizes useful practical theoretical material and methods for studying both thin sections of rocks and immersion of mineral grains in refractive index liquids. It contains original research results found in no other text. A major goal of the text is to allow precise determination of refractive index and the essential composition of crystals belonging to important mineral groups such as olivine, feldspar, and pyroxene. New methods for achieving this are developed for both white light and colored light of variable wavelength. Among the book's unique features is the color fringe chart developed by Prof. Morse for estimating both the direction and degree of mismatch between the refraction index of a crystal and that of the surrounding liquid medium in the immersion method. Further, a new algebraic treatment of the dispersion method allows a high precision of match between crystal and liquid. An original classification of interference figures aids crystal identification. Worked examples of refractive index determination and crystal identification are given for each optical class of crystals. The optic orientation of optically biaxial crystals is illustrated with examples from each crystal system portrayed in stereographic projection. Principles and applications of crystal identification with the dispersion method are developed in aseparate chapter. The final chapter is a practical, step-by-step guide to crystal identification in thin section or immersion. An identification table for the most common asbestos minerals, including the dispersion staining method used by most environmental laboratories.