Flint

Flint is an expendable object found by quarrying gravel. When destroying gravel blocks, there is a 10% chance that a single unit of flint will drop instead of receiving the gravel block back. Some players farm flint by erecting high stacks of gravel and then power-mining it down. Note however that this is a one-way process: there is no way of turning flint back into gravel.

Gravel, along with other blocks that are affected by gravity, can be harvested quickly by placing a torch under a tall stack of it, then breaking the gravel immediately above it. The gravel blocks will break when they hit the torch, causing the entire stack to quickly break down. Using this tactic, however, will not yield flint; you must manually mine gravel with a tool or your hands if you wish to increase your supply.

Trivia

 * If a gravel block falls onto a torch from any height, it will break, but has no chance of creating flint, and this does not happen on ladders and many other blocks. (Note also that gravel will only do this if falling, allowing torches to hold gravel up in place when done correctly.)
 * After the 1.3 update, flint is more common to find.
 * Flint is the rarest thing that is required to create arrows, so never ever waste it.

Krzemień Feuerstein