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.

The face of a block is 16x16 pixels, with each block estimated to be one cubic meter. Water and Lava blocks have a shifting pattern for each face.

Types of Block
There are currently over 55 different types of blocks (including Air,  still Water, still Lava, blocks which cannot be built at the  moment, (see Blocks, items and data values for the full list) and  assuming each Cloth color as an unique block.)

In a standard randomly generated infdev Map you can find the blocks  Stone, Grass (or Snow), Dirt, Water, Lava,  Sand, Gravel, Gold ore, Iron ore, Diamond,  Redstone, Clay, Coal, Tree, Leaves, Cactus,  yellow and red Flowers and brown and red Mushrooms.

In the initial free version of Survival the player began with 10  TNT blocks and could obtain Dirt, Cobblestone from Stone,  Wood 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 Mossy Cobblestone. Operators of servers can also build Bedrock, aka Adminium. Some custom servers give the ability to place Grass and (still) Fluids, too.

When playing the current Alpha, the player can craft a  variety of blocks not naturally found in maps, including Stairs,  Workbenches, Paintings, Bookshelf and  Furnaces. Redstone and Diamond can be found in the lower areas of maps, and Obsidian forms when water touches a spring  lava block.

Also in the current Alpha, the Mob Spawner block may naturally occur in a Dungeon.

Notch and PC Gamer have confirmed that the Halloween update will add 6 new blocks, 5 of which will serve its own new function. In addition, Obsidian will be able to be used as a portal between the new Hell dimension. It is not yet known if this new use for Obsidian counts as one of the new 6. Two of the blocks have been revealed by PC Gamer through screen shots directly from Notch. One block appears to be a new red version of Mossy Cobblestone* which makes up the majority of the Hell dimension. This block will continue to burn when set alight until doused by a player, making them perfect for traps. The second block appears to be a gold version of Cobblestone. Dubbed "Australium" after the gold-like substance from Team Fortress 2 by PC Gamer, the block can be seen on the roof of the Hell dimension, and may be used as a new crafting material for tools, or as a light source, also used to make Lanterns.


 * *Note:This was confirmed by Notch on twitter.

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