Gem opals, from volcanic host rocks from a variety of sources, have been examined by electronmicroscopy and diffraction. They are generally a mixture of amorphous and crystalline silica, the extent of crystallinity varying between samples from different localities. The crystalline phase in some samples has been identified as tridymite. Their microstructures are compared with those of gem opals from deposits in sedimentary rocks, and with specimens heated in the laboratory. Changes in morphology produced by sintering occur at about 400°C, and crystallization at about 1100°C. Both tridymite and cristobalite were identified in material recrystallized by heating.