Chert is a sedimentary rock made out of many fine grains of a mineral compound called silicon dioxide. It is a variety of cryptocrystalline quartz.
In clear quartz, the silicon dioxide particles that make up the stone are chained together into one large, unified crystal. The unified nature of the chains gives the stone a translucent sheen.
In chert, the silicon dioxide is broken up into many small, individual crystals that merge together into one continuous layer of stone. This makes chert relatively opaque and dull compared to quartz.
Chert comes in many different colors, including white and green. Flint is chert that is especially dark.