With this piece of flint and a fire steel (strike-a-light, flint striker) it is easy to produce sparks. To start a fire, these need to land on a suitable tinder – that's th medieval way!
To work properly the stone needs at least on sharp edge, so it may have to be treates with a hammer or stone (wear safety goggles!).
Dimensions: ca. 4 x 5 cm, weight: ca. 80 g. Shape, dimensions, and weight may vary, since this is a natural product.