Chert is a mineral. It can be very difficult to distinguish between chert and flint, which
are both forms of nodular silica. Flints formed in the soft limestone formation called
the Chalk. The Chalk is a calcium carbonate rock formed from the remains of minute
planktonic organisms called coccolithophores in a sea which covered most of
Northern and Central Europe and Britain during the Late Cretaceous, 100 to 65
million years ago. Chert is often described as the mineral which corresponds to flint
in limestones other than the Chalk. Chert can also be found in sandstones. Whereas
flint is formed as nodules (individual lumps), chert can be formed as nodules or large
beds (layers in the rock).
There is little if any chemical difference between flint and chert. Both consist of
extremely small crystals of quartz (silica) tightly packed together. In the flint of the
Chalk the silica has come sponges. The familiar bath sponge is the skeleton of a
sponge with a flexible protein skeleton containing no mineral matter. However, many
sponges have skeletons made of tiny silica rods called spicules. Such sponges were
very common in the chalk sea and their skeletons were preserved in the Chalk. This
was subsequently buried under later deposits, together with some of the water
contained in it, and the conditions of higher pressure and temperature which
accompanied this burial the water dissolved many of the spicules. Flint nodules were
formed subsequently by the redeposition of this silica, generally replacing some of
the calcium carbonate which forms the Chalk. The silica that forms chert may have
come from sponge spicules but also from other organisms with siliceous parts such
as single-celled diatoms or Radiolaria.
Flint and chert are hard and insoluble, so they are left behind when the Chalk or
other limestone has been dissolved and weathered away. The nodules are broken up
by frost, stream or wave action, and the last two of these wear the fragments into
round pebbles. Flint brakes with a concave or “conchoidal” fracture but the fracture in
chert can be more splintery. Freshly broken flint is dark and smooth and weathers to
a rough white crust or ‘cortex’. Chert does not form such a well marked cortex but
some translucent cherts do become opaque towards the surface.
Flint and chert sometimes have fantastic shapes with a superficial resemblance to a
variety of things, such as bones, skulls, claws or teeth. The nodules are sometimes
long and hollow and when broken may be mistaken for the bones of land animals. All
such shapes are accidental but some flint and chert nodules have actually have
formed around fossils such as sea urchins, shells and sponges and preserved them.
References and Further reading:
Shepherd, W. 1972. Flint: Its Origin, Properties and Uses. London, Faber and Faber.
255 pp + 32 plates.