Block

Minecraft is composed of and built entirely out of blocks. You dig through blocks, you build with blocks, your character is made of blocks; they are everywhere. It is possible to change how the blocks appear by applying new Texture Packs.

The face of a block is 16x16 pixels, and each block is proportionately one cubic meter. Water, Lava and Portal blocks have a shifting pattern for each face.

Types of Blocks
There are currently 91 different types of blocks (including Air, still Water, still Lava, blocks which cannot be built at the  moment, (see data values for the full list) and  assuming each Wool color as a unique block.)

In a standard randomly generated Beta Map you can find the blocks Stone, Grass (or Snow), Dirt, Water, Lava, Sand, Gravel, Gold ore, Iron ore, Diamond, Redstone, Lapis Lazuli ore, Clay, Coal, Ice, Tree, Leaves, Cactus, Sugar Cane, yellow and red Flowers and brown and red Mushrooms. In dungeons (added in Seecret Friday Update 2), you can also find Cobblestone, Moss Stone, a Mob Spawner, and Chests, even though cobblestone and chests are usually made by the player. Obsidian can be found naturally, but it is not part of the random generation, instead it is created when naturally occurring lava source blocks and water come in contact.

In the initial free version of Survival the player began with 10 TNT blocks and could obtain Dirt, Cobblestone from Stone, Wooden Planks from Trees, Gold from Gold ore, Iron from Iron ore, Step from Coal, and white Cloth  from Sheep.

In Classic mode the player can build with naturally occurring blocks, save Redstone and Diamond, and can use Bookcase, Sponge, Coloured Cloth, Brick, Obsidian and Moss Stone. Operators of servers can also build Bedrock. Some custom servers give the ability to place Grass and (still) Fluids, too.

When playing the current Beta, the player can craft a variety of blocks not naturally found in maps, including Stairs, Workbenches, Paintings, Bookshelves, Furnaces and more. Redstone, Diamond and Lapis Lazuli can be found in the lower areas of maps, and Obsidian forms when water touches a spring lava block. The Mob Spawner block may naturally occur in a Dungeon. Pumpkins can grow on the surface.

The blocks Netherrack, Glowstone, and Soul Sand can be found in The Nether, which the player can only enter by creating Portal blocks.

Opacity
Opacity is a property of blocks. It determines ray tracing and collision result and controls several interaction behaviors.

Translucent Blocks

 * Plants: sapling, yellow flower, red rose, brown mushroom, red mushroom, crops, sugar cane.
 * Liquid: water, stationary water, lava, stationary lava.
 * Blocks: leaves (opaque when graphics are on fast), glass, step, mob spawner, wooden and cobblestone stairs, soil, snow, ice, cactus, fence.
 * Non-blocks: torch, fire, redstone wire, sign post, wooden and iron door, ladder, rails, wall sign, lever, wooden and stone pressure plate, redstone torch off/on, button, portal.

Behaviors

 * Snow, doors, buttons, torches, ladders, levers, minecart tracks, and pumpkins can only be placed on opaque blocks.
 * Mobs only spawn on opaque blocks and in transparent/open space.
 * Fireballs only start fire on opaque blocks.
 * Mushrooms only grow on opaque blocks.
 * Growing leaves no longer destroy any translucent blocks.
 * Opacity of adjacent blocks determines the direction of furnaces and chests.
 * Chests can't be opened if there are opaque blocks above.
 * Translucent blocks are not ignitable.
 * Translucent blocks change water behavior.
 * Block opacity affects the behavior of redstone electrics.