Flint is a form of quartz (SiO2), which is relatively hard and rather easy to sculpture into a certain form. The result are also sharp edges which are very good for cutting things. It was widely used during the Stone Age, that's why this time is called Stone Age. The flint was rather common in certain areas, especially where layers of Cretaceous chalk exist in northern Europe and southern Great Britain. The flint is a concretion which was formed by ground water processes in certain layers. The nodules have different sizes, up to half a metre in diameter.
On the other hand, good flint of a high quality was found only at certain places. It was collected at this places, hewn into bars and traded with remote areas.
It is known that Stone Age man collected very fine examples of flint and took them with him. They were transported for several hundred kilometers. Especially to the end of the Stone Age, during the Neolithic or New/Late Stone Age, the flint was mined in the first mines of humankind. Starting with quarries and open cast mines, soon shafts were built to find the flint layers. The chalk is typically very soft, and so man was able to cut it with his stone tools. The flint mines differed depending on the surface landscape, the kind of rocks and the morphology of the flint deposit.
The size of the mines shows, that mining soon became a job for specialists, for miners, who spent their whole life searching for this valuable material. Some details even show that they had high skills in their job. So the miners at Spiennes cut through a rather hard layer of limestone to reach another layer of chalk with valuable flint. It seems they knew enough of the geology to know that their would be flint below, which would reward the enormous effort.
Flint was used all the time for building houses or streets. There are medieval flint mines, which are not called so as these quarries were used to quarry any kind of rocks usefull for buildings. Today flint is not any more valuable, but still it is mined. The mines of today are much different to both the Medieval and the Neolithic ones. It is mined in huge amounts, preferably in rather fine grains which do not need much processing. They are used as a hard component of concrete, and so they must be rather small. And there are many other uses for quartz like producing quartz based computer chips. But the quartz used for this is typically fine quartz sand, not the nodules from the Cretaceous chalk.