Sand

Sand is a block affected by gravity.

Red sand is a variation of sand that cannot be used to craft concrete powder, and on which turtles cannot hatch.

Natural generation
Sand generates naturally in many biomes of the Overworld near lakes or pond, notably deserts, beaches, and rivers, generally in four-block-deep layers supported by stone and sandstone. Sand also generate as ocean floor in lukewarm, warm ocean and their deep variants

Sand is also used to cover up buried treasure chests depending on where the chest generates in. (In this case, sand is used to cover up buried treasure chests that generate in beaches and ocean floors that are composed of sand, but in some cases stone or sandstone might be used instead).‌ [Java and Bedrock editions only]

Sand also generate as part of warm underwater ruins and some desert village houses.

Red sand generates naturally in badlands biomes and variants.

Both types of sand can spawn floating in the air. The floating cluster of sand falls when one of the sand blocks receives an update (when a block is placed near it or when a block near it is broken).

Usage
Sand, if there is no block below it, falls until it lands on the next available block. While sand is falling, it exhibits a smooth falling animation.

If falling sand lands and covers the head of a mob or the player, the mob or player buried in it continuously receives suffocation damage. If falling sand lands in the space occupied by a non-solid block (such as torches, slabs, rails, or redstone), it drops and turn into an item. If it falls onto a cobweb, it falls slowly until it has gone through completely; if it touches the ground while still inside the cobweb, it becomes an item.

Sand can be placed on a non-solid block without falling.

ID
Java Edition: Bedrock Edition:

Issues
Issues relating to "Sand" or "Red Sand" are maintained on the bug tracker. Report issues there.

Trivia

 * If a block of sand has snow on it and is made to fall, the snow is destroyed‌ [Java Edition only]  and does not drop snowballs.
 * Since falling sand is considered an entity, it can be launched in a TNT cannon, similar to an ignited piece of TNT.
 * Sand falls at the same rate when submerged in water, or lava, or air.
 * If a player is standing on a stack of sand or gravel, and the stack falls onto a non-solid block, the player can fall fast enough to take damage or even die.
 * Sand falls through torches without breaking, if there is air below the torch.
 * Sand and gravel take about 0.45 seconds to fall one meter.
 * The sand texture is rotated if a block of sand turns into a FallingSand entity, similar to ignited TNT blocks. This is not the case with gravel.
 * Since falling_block is considered an entity, blocks of sand without supports may disappear for a split second and a falling_block entity is summoned. This is also the case when it's landing, but the entity (not the block) disappears instead before a block is placed.