Flint

Flint is raw material used for creating arrows and flint and steel. It is found by mining gravel. When mined, there is a 10% chance that a single unit of flint will drop instead of a gravel block. Flint can also be obtained by trading gravel and emeralds with villagers.

Farming
You can easily gain flint in huge quantities by collecting gravel. Collect as much gravel as you want to obtain via flint and proceed to a safe, well-lit location (as this is a good thing to get done during the night) and place the gravel in a sufficiently large block (2 or 3 high is adequate). With a shovel equipped, dig through all of the placed gravel picking up any gravel/flint that drops. Rinse and repeat until you have used up all of your gravel. This technique can be done in proximity to a Mob Spawner trap, allowing their 'loot' to build up while you dig. Gravel blocks drop flint 10% of the time, however using a tool enchanted with Fortune increases the chances that flint drops.

Another technique is to either outside, or inside if you have a place big enough, look straight down and keep placing the gravel right beneath you. Once you place all of the gravel, simply dig straight down. Using this technique will cause less mistakes and be quicker since they are in a perfectly straight line. However it may be a challenge to find a space big enough to do indoors or be dangerous to do it outside at night.

Another thing you can do is make a vertical shaft. This technique is almost like the above one except that this one you can easily get to the top and bottom and can therefore add gravel to it as you collect it and mine it at time (a good idea is to drop off gravel during the day and mine what you have collected at night). It should look something like this:

This can be as tall as you want it and gives the added bonus of getting some stone and possible ores as you clear out an area to make it.

Flint is only obtainable by mining it yourself or exploding the gravel; making gravel fall onto a non-solid block (e.g. torch, sapling) does not produce flint.

Trivia

 * Gravel mined with a Silk Touch enchanted item will never produce flint under any circumstances.

Feuerstein Flint/es Silex Kovakő Vuursteen Krzemień pt-br:Flint Кремень 燧石