Flint
Flint is a cryptocrystalline (means you can't see the crystals) form of the mineral quartz. It is a variety of chert. It occurs as a line of knobbly masses in  chalk and some limestones. Inside the nodule, flint is usually translucent, and can be various colours. A thin layer on the outside of the nodules is usually different in colour, typically white and rough in texture.