Agate is a rock consisting primarily of cryptocrystalline silica, chiefly chalcedony, alternating with microgranular quartz. Agate is a translucent variety of microcrystalline quartz. It is used as a semiprecious stone when it is of desirable quality and color. Agate generally forms by the deposition of silica from groundwater in the cavities of igneous rocks. These structures produce the banded patterns that are characteristic of many agates. Agate occurs in a wide range of colors, which include brown, white, red, gray, pink, black, and yellow. The colors are caused by impurities and occur as alternating bands within the agate. Agates develop as secondary deposits in hollow cavities, called vesicles. Although they can form in all types of host rock, most of the world's agates developed in ancient volcanic lava. ... Later, these gases escaped through cracks that formed as the igneous rock cooled and har