Chert is a fine-grained (microcrystalline) variety of quartz. It is also the name given to rock composed primarily of fine-grained quartz. So, the name is used in two ways. Chert (the silica mineral kind) may be massive or layered. It is often in nodules or concretions in limestone. Some chert forms by recrystallization of amorphous silica. Chert has many appearances; the five photos below (Figures 7.59 to 7.63) show examples of some of the more common varieties. Common chert is light grey. Jasper is chert with a characteristic red color due to hematite inclusions. Flint, a darker form of chert, contains organic matter. Opal and chalcedony, two other types of silica, are often associated with chert deposits. From the Stone Age until the Industrial Revolution, chert and flint were highly valued for use as weapons, tools, and fire starters.