Gravel

Gravel is a type of block typically found in naturally occurring pits, underwater, or underground. Like Sand, Gravel will fall to the lowest z-coordinate below it if there is no block underneath it. Therefore it is possible to suffocate with careless use of gravel by being crushed beneath the weight. If the lowest block in a column of Gravel is occupied by a Torch or flower, the Gravel block will disintegrate into a gravel resource instead.

In Alpha, it has a roughly an 8.5% chance to drop Flint once destroyed, which is used to create Flint and Steel and Arrows. Shovels will destroy this block quicker than normal, like Sand and Dirt. Due to the fact that Gravel is affected by gravity, it was used to make hallways that seal when water touches torches supporting it, creating an Airlock.

There was a glitch in an older version of singleplayer Creative mode that caused a flood if water was placed on a wall with blocks affected by gravity; this bug has since been fixed.

Minecarts prevent Gravel from falling.

Trivia

 * On occasion, some normally generated maps may produce beaches made of gravel instead of sand.
 * A great way to power mine massive columns of gravel is to dig under the stone or dirt that it is resting on. Under that dirt or stone, place a torch. Mine the dirt or stone, and watch as the gravel column falls into the torch quickly, giving you gravel blocks. This also works with sand. This does not appear to yield flint, however. 