Gravel

Gravel is a type of block typically found in naturally occurring pits, underwater, on beaches, NPC Villages as pathways, or in The Nether. It is generated at a rate of approximately 3.29% that of stone between layers 1 and 111.

Like Sand, Gravel is affected by gravity and will fall to the nearest solid block below it. Therefore it is possible to suffocate with careless use of gravel by being surrounded by 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 a resource entity when it hits the partial block.

Gravel has a 10% chance of dropping flint once destroyed. 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 considering the other gravity-aware blocks, Sand, is more useful for creating Glass, TNT, and Sandstone, the Enderdragon egg is only found once, so using it would be a waste of time and effort. For exploration, it can be used to quickly build pillars to reach heights that are easy to dismantle after. They can also be used to quickly fill in water or lava lakes by dropping them on the edges or against overhead blocks, so that they fall and occupy the fluids. Filling caves in with gravel is a quick way to prevent mobs from spawning. Mobs can be suffocated with gravel, too.

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

Gravel, or sand can be used to help with obsidian mining, though it is somewhat time consuming. 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 you place. When the gravel reaches the top, start filling in parts of the lake as normal, stopping one block below the surface. Then create a water spring to form the lava into obsidian. You will be able to mine the obsidian safely now that the gravel is under it. 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.

Power-mining
Like Sand, Gravel will fall to the lowest y-coordinate below it if there is no solid block underneath it. Therefore it is possible to suffocate with careless use of gravel by being surrounded by 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 a resource entity when it hits the partial block. A great way to power-mine massive columns of gravel is to dig under the stone or dirt that it is resting on and place a torch, a redstone torch, a piece of redstone wire, a pressure plate, a section of rails, a sign, a slab or a trapdoor. Mine the dirt or stone and the column falls into the placed object, quickly producing dropped resources. Note that this only produces gravel items; gravel is recommended to be mined by hand to produce flint. This approach can also be used to mine Sand. Gravel can also be harvested at a great speed when using an iron or better shovel on a stack with more than 2 blocks on it. The reason it is so fast is because the gravel is mined faster than it drops, allowing it to be harvested as fast as if there were a torch under it, but still having the chance to make flint while mining. This is useful when reusing gravel in an attempt to get flint. The best position to do this is with your cross-hairs pointing at the upper portion of the bottom-most block in a stack.

Descending caverns and ravines
Gravel can be used to descend a cavern or ravine as well as just creating a staircase so that if you dug in through the ceiling, you can get back out. If you place gravel over one tile until the column reaches the top, and then dig down through the column, you can quickly descend pits, and ravines. Once you reach the bottom, though, you should mark the bottom with something such as four torches surrounding it so that you can use the gravel to leave. To do that, just place gravel below you while jumping up. This is especially useful when entering a stronghold because you can destroy the silverfish spawner without the silverfish attacking you and pushing you away.

Bugs

 * Floating gravel - sometimes, gravel and sand can be found suspended above the ground without being affected by gravity. You can stand on the gravel without it falling, but if you alter any block adjacent (on any of the 6 sides of the block), the block, as well as the blocks adjacent to that block will fall. This continues to happen for all the blocks directly adjacent to the glitched gravel or sand block. If a block is diagonal to this block, it will not affect the block.
 * When gravel blocks are on top of each other, when the bottom block is broken, the gravel above falls down and the texture becomes lighter.
 * When mining gravel with an enchanted pickaxe or a shovel enchanted with Fortune level 3 or above, it will cause the game to crash with a "Saving Chunks" screen.

Trivia

 * In the Nether, it is possible to find massive cliffs made of naturally-floating gravel. Just like with Sand, if you destroy, replace, remove, 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 is usually a two-high block wall upland from the beach.
 * Gravel beaches on map are drawn with the same color as sand beaches.
 * It takes, on average, 12 wooden shovels to convert a stack of gravel into flint.
 * If gravel 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, and Bedrock 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.
 * Often, gravel will 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.
 * In the Nether, gravel will mostly be found naturally at layer 63, 64 and 65.
 * If you blast gravel (Try in an NPC village) with TNT, the blast may move the gravel without destroying it, similar to mobs or players.
 * Falling gravel, like other entities, will fall faster than the player.

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more info

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Gravel is the derpeyest block of all time.