Regular flint. Few words about.
Flint (or flintstone) is a hard, sedimentary cryptocrystalline silicate form of the mineral quartz, categorized as a variety of chalcedony. Flint is usually dark-grey, blue, black, or deep brown in colour, and often has a glassy appearance. It occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones.
The exact mode of formation of flint is not yet clear or agreed but it is thought that it occurs as a result of chemical changes in compressed sedimentary rock formations, during the process of diagenesis. One theory is that a gelatinous material fills cavities in the sediment, such as holes bored by crustaceans or molluscs and that this becomes silicified. This could certainly explain the complex shapes of flint that are found.
source: wikipedia.org