Flint is a hard,
sedimentary cryptocrystalline form of the
mineral quartz, categorized as a variety of
chert. It occurs chiefly as
nodules and masses in sedimentary rocks, such as
chalks and
limestones. Inside the nodule, flint is usually dark grey, black, green, white or brown in colour, and often has a glassy or waxy appearance. A thin layer on the outside of the nodules is usually different in colour, typically white and rough in texture. From a
petrological point of view, "flint" refers specifically to the form of
chert which occurs in chalk or marly limestone. Similarly, "common chert" (sometimes referred to simply as "chert") occurs in limestone.