A geode is an almost spherical hollow body, the exterior of which is usually composed of a fine-grained variety of quartz such as agate. The inner surface is generally covered with crystals. The crystals, which project inward toward the center of the geode, may be amethyst or another gem variety of quartz. Geodes which may 'be as small as an inch or larger than a foot in diameter, occur in sedimentary rocks such as limestone.
