Flint (flint) tumbled stones from Greenland.

Flint or cobblestone, flint or flint ("paper-thin") is a rock that is often found in clumps in limestone and is usually brown or gray in color. Such 'lumps' are called 'concretions' in geology. The flint concretions deposited in the Late Cretaceous period are very rich in shape and vary from elongated plates to antler- and bone-shaped stones. Flint consists of cryptocrystalline silicon dioxide and a lot of (chemically bound) water. It is a very hard (7 on the Mohs hardness scale), clastic rock that is mainly found in filled crustacean burrows. A hard blow to a piece of flint often creates a shell-shaped fracture surface with sharp edges. This is caused by a stress wave in the rock due to the impact. The rock is called flint because a blow with a piece of flint on a piece of iron or pyrite can result in sparks, which, with the necessary experience, can ignite a dry, flammable material (such as a tuft of loose cotton or dried moss, or tinder fungus) become. The sparks are caused by small iron particles that spontaneously oxidize in the air, releasing a lot of heat so that the particles glow. In prehistoric times (the Stone Age), people such as Neanderthals and Cro-Magnon man made stone utensils, such as scrapers, arrowheads, axes and blades, preferably from this material. The age of the object can be estimated fairly accurately based on the degree of refinement of the product and the techniques used. Because such utensils, unlike human and animal tissue, wood and textiles, do not decay, they are virtually the only traces we have of prehistoric man, apart from processed or unprocessed bones and potsherds.