Chert is a fine-grained siliceous sedimentary rock. It is a hard, dense, and consist chiefly of interlocking microscopic crystals of quartz and may contain opal. It has a conchoidal fracture and may occur in a variety of colors. Most chert forms from recrystallization of siliceous microplankton remains (siliceous ooze eventually looses its water content, recrystallizes and turns into chert.
Organic residues preserved as chert beds are known from rocks dating back to early Precambrian time. Banded-iron formations (BIFs) are composed of interbedded layers of iron-oxide minerals and chert, and are thought to be biogenous in origin. Younger marine cherts are mostly formed from diatoms and radiolarian oozes.
|Figure 6.85. Layers of marine chert exposed in the Marin Headlands, California||Figure 6.86. Banded-iron formation (with chert) from the Precambrian era.|