A teeny, tiny particle of something is a granule. If ants love to invade your kitchen, you'll need to carefully clean up every last granule of the sugar you spilled on the floor.
In geology, a granule is actually a specific, measurable amount. It's between two and four millimeters — larger than a grain of sand, but smaller than a pebble. In everyday use, a granule is just a very small piece of something. If your little brother is especially sensitive, he may insist on periodically stopping during a hike to empty granules of gravel out of his sneakers. Granule comes from the Late Latin granulum, "small grain."