This rock is made up of cryptocrystalline silica (quartz), black to brown. Being made up of silica, it is hard and displays conchoidal fracture.

This is a flint nodule, which comes from a chalk bed – notice the thin shell of white chalk. Flints are irregular silica concretions characteristic of the upper Chalk.

Cryptocrystalline silica (quartz) 
Amorphous, not grains. Partially devitrified 
Uniform texture 
Rare larger quartz crystals

Occasional elliptical strips of greater than average grain size probably represent cross-sections of shells now replaced by silica.

Flint nodules are produced by the digenetic replacement of chalk. A silica gel, formed by the dissolution of sponge spicules, precipitates silica in nodules. Chert deposits are more common in organic-rich regions of the chalk, and there is a hypothesis that the silica precipitates in burrows.