The Piece of Flint is one of the items that are necessary to solve the riddle in Arqua. Together with the Steel Cylinder we can create a spark, allowing us to enkindle the pack of combustible Tinder which eventually, is representative of the Fire element.
The second use for this object is that, banging the combined Flint and Cylinder against any solid object in our inventory, such as the Hookah Pipe, the Bowl or Flask, the resulting sound will open the niche with the "Bong" tile, the hidden stash of the Amber Gem.
Adam: Piece of Flint.
Flint is a hard, sedimentary cryptocrystalline form of the mineral quartz, categorized as a variety of chert. It occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones. Inside the nodule, flint is usually dark grey, black, green, white or brown in colour, and often has a glassy or waxy appearance.
When struck against steel, a flint edge will produce sparks. The hard flint edge shaves off a particle of the steel that exposes iron which reacts with oxygen from the atmosphere and can ignite the proper tinder.
Darren Bush has detailed the methodology of igniting a fire with flint and steel in his tutorial Traditional Firestarting Part I: How to Make Fire with Flint and Steel.
A later, major use of flint and steel was in the flintlock mechanism, used primarily in flintlock firearms, but also used on dedicated fire-starting tools. A piece of flint held in the jaws of a spring-loaded hammer, when released by a trigger, strikes a hinged piece of steel ("frizzen") at an angle, creating a shower of sparks and exposing a charge of priming powder. The sparks ignite the priming powder and that flame, in turn, ignites the main charge, propelling the ball, bullet, or shot through the barrel.