Gravel

Gravel is a type of block typically found in naturally occurring pits, underwater, on beaches (more commonly in older versions of the game), NPC Villages as pathways, or in The Nether. In the Overworld, it is generated at a rate of approximately 3.29% that of stone between layers 1 and 111. In the Nether, gravel will mostly be found naturally at layer 63, 64 and 65.

As a FallingSand entity, gravel is affected by gravity and will fall to the nearest solid block below it. Players and mobs will suffocate under gravel. If the space beneath falling gravel is occupied by a partial block, such as a Torch, the gravel will drop as an item.

Gravel blocks are the only source of flint, which has 10% chance of dropping when gravel is mined. Flint is used to create Flint and Steel and Arrows. Gravel blocks that don't drop flint can be picked up, placed, and destroyed again. However, if a gravel block drops flint, it does not additionally drop a gravel block, preventing the creation of an infinite amount of flint. Flint can be obtained from gravel with any tool. Flint can also be obtained by using TNT on gravel. Use of the Fortune enchantment will improve the chances of flint.

Usage
Gravel's abundance and gravity-obeying property makes it useful for a variety of tasks, especially since Sand has other uses. Filling caves in with gravel is a quick way to prevent mobs from spawning, but a lot of gravel is needed.

Gravel can be used to create a suffocation trap.

Gravel can also be used to make airlocks (sand-switches). Gravel is a bit less noticeable underground than sand.

Gravel can be used to help with obsidian mining, though it is somewhat time consuming and wastes lava. Locate a natural lava lake, and start to fill it in with gravel as if to get rid of it, being careful to count the amount of blocks needed to reach the top. The rest of the lake can be filled to one less than that. Once the lava becomes obsidian, it can be safely mined; there shouldn't be any lava underneath. This method is somewhat time consuming, and also can be a little hit and miss while placing the gravel, as the bottom of the lake might not be completely flat.

Gravel can be used to easily descend a cavern or ravine. By placing enough gravel at the top of a hole, it will eventually pile up to the top; at that point it is safe to mine the stack directly downwards. Gravel columns can be quickly removed by placing a torch under the bottom most gravel block. This makes gravel useful for pillar jumping.

Gravel is not commonly used for decoration. Due to its dull color and the fact it must be supported by another block, it has few advantages. It is often used underneath rails to mimic real railroads, since a layer of gravel can be found underneath rails. It also can be used as cheap replacement for iron blocks in building because of its somewhat metallic texture.

Power-mining
Since gravel will fall to the lowest y-coordinate below it if there is no solid block underneath it, if the lowest block in a column of gravel is occupied by a partial block, such as a Torch, the gravel block will drop and turn into an item when it hits the partial block. This is an effective way to harvest stacks of gravel, although flint can only be obtained by actually mining gravel. Gravel can mined at a great speed when using an iron or better shovel on a stack with more than 2 blocks on it; because the gravel is mined faster than it drops, this is as fast as dropping it on a partial block. This is useful when reusing gravel in an attempt to get flint. The best position to do this is with the cross-hairs pointing at the upper portion of the bottom-most block in a stack.

Trivia

 * In the Nether, it is possible to find massive cliffs made of naturally-floating gravel. Just like with Sand, if one block of the gravel is destroyed, another block is placed adjacently, or if a Ghast's fireball hits any of these blocks, all of the adjacent floating blocks will collapse.
 * Before Beta 1.8 terrain generation, on gravel beaches, there was usually a two-high block wall upland from the beach.
 * It takes, on average, 12 wooden shovels to convert a stack of gravel into flint.
 * If a gravel FallingSand entity falls into lava, it will burn as if it was an item, although it will not disappear.
 * If gravel falls into a cobweb, it will be slowed down and then converted to an item.
 * Gravel, Obsidian, Bedrock, Lava, (and as of Title Update 9 on the Xbox 360 edition iron bars) are the only blocks that naturally spawn in multiple dimensions.
 * If a player is standing on a stack of sand or gravel, and the stack falls on a non-solid block, the player will fall fast enough to take damage or even die.
 * Gravel will often fall into caves making a mock dead end. Thus, if a player encounters a gravel dead-end while mining, removing the gravel may reveal additional passageways.
 * Sometimes, over-hanging lands in extreme hills biomes will be formed with gravel as a bottom layer, and this normally falls to the ground below, leaving gravel patches on the ground.
 * If you blast gravel with TNT while it is falling, then instead of being destroyed it will be forced away, like a mob or player.
 * If you light gravel on fire, it no longer obeys the laws of physics. The same applies to sand.
 * Falling gravel, like other entities, will fall faster than the player.
 * On a map, gravel looks yellow, making it easy to mistake gravel for sand, although in the Nether, it is known that it is gravel, not sand.
 * Like sand, gravel is affected by gravity in Pocket Edition.
 * In real life, a pickaxe is often a better choice than a shovel for digging through compacted gravel or metal.
 * Because of the ability to reset the Nether in the Xbox 360 and PS3 edition, gravel is renewable in it.
 * Along with Sand, Gravel has existed in the Far Lands prior to Beta 1.8, as multiple falling entities, causing extreme lag.