Minecraft wiki entry for Flint, associated text: When a block of gravel is mined, there is a 10% chance for a single piece of flint to drop instead of the gravel block. When mined with a Fortune-enchanted tool, this chance increases to 14% at Fortune I, 25% at Fortune II, and 100% at Fortune III. Gravel mined using a tool with Silk Touch or gravel that fell on a non-solid block never produces flint.