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7 October 2020: FlintA piece of flint

Ros Mercer, the Secretary of the Essex Rock and Mineral Society spoke to local National Trust Supporter Group online about flint. This may not seem like a very interesting subject. After all, flint is a very common mineral in this area and flint pebbles can be found on many beaches and in our own gardens. However, Ros gave us some interesting insights into this fascinating material.

Flint is only found in chalk deposits, of which we have an abundance thanks to the “chalk sea” that covered much of Europe in the Cretaceous era, around 100 million years ago. The chalk was formed from the remains of the shells of tiny plankton. The flint layers formed from the remains of some of the sea creatures of the time, particularly sponges.

Flint can be found in a variety of interesting shapes. Some of these arose because the remains of sea creatures found their way into small tunnels formed in the chalk sea floor by shrimps. The round flint pebbles we see were probably originally of different shapes but over millions of years rounded by the action of sea and rivers.

Flint has a crystalline structure formed of a mixture of opal and chalcedony. It contains micro pores, into which other chemicals can make their way. Most flints have a black core, formed of minute crystals that do not reflect the light, and a whitish outer coating caused by the interaction of the flint with the chalk in which it was embedded. If you find a red flint it is evidence that it was at one time in a natural or human-made fire; the extreme heat forced the iron in the micro pores to turn into haematite. Polished flint can be very attractive. It has a similar crystalline structure to glass.

Whilst we may think of Stone Age humans as very primitive, they were in fact very skilled at working with flint to turn it into the tools they needed. Based on modern attempts to recreate their craft, it is thought they would hit a flint and listen to the sound to determine where best to strike it to cause a break. Flint can take a fine edge.

Flint is still used as a building material in some places. Ros showed us some photographs of knapped flint used a surface for church walls.

She had some examples of flint with her and held them up to her camera so we could all see them. These included two halves from a split flint pebble where the exposed surface had been polished and showed the remains of a sponge. Her husband had bought one half and she had later bought the other. It wasn’t until they looked at them together they realised that they were two halves of the same stone.