Diamond Ore

Diamond Ore is generally considered to be the most valuable and elusive block in the game. It can be mined for a diamond gem using an Iron or Diamond Pickaxe.

Uses
Diamonds can be used to make Shovels, Pickaxes, Axes, Swords, Hoes, and Armor of the highest quality. It is also a prequisite ingredient in Jukeboxes, solid diamond blocks, and Enchantment Tables.

Prospecting
Diamond Ore can be found in veins of 1-10 blocks (sometimes even more if multiple veins happen to spawn together, and down to 1 if the block that was supposed to be diamond ore is occupied by a cavern, dungeon, lake etc.). Diamond only appears in the bottom 16 layers of the map.
 * Diamond ore occurs in roughly 0.1276% of stone from levels 2-14.
 * There is an average of 3.097 diamond ore per chunk.
 * Always dig around a diamond block before mining it to ensure that the gem won't fall into a cave, lava, etc.

Trivia

 * When diamond was first introduced, it was called emerald.
 * Diamond ore can be smelted into a diamond gem, although the only way to obtain it is with the silk touch enchantment. It could be smelted even before other then-unobtainable ores could be.
 * Diamond, like other ores, is easier to find in caves and empty lava pools, as this gives more surface area.
 * Before Beta 1.6, diamond was about 50% more common in the quadrant south-west of the point (0,0) compared to the quadrant to the north-east.
 * In some vein formations, ores connect diagonally. Therefore it is recommended to mine around diamond ore. This also permits checking for lava.
 * Diamond ore, along with other ores, appears in the background on the achievement page.
 * Diamonds are considered to be the rarest item in the game. However, prior to 1.8 Clay was actually rarer, though much easier to find. Likewise, in 1.0.0, End Portal Frames are rarer, but still easier to find.
 * Diamond ore cannot be found or placed in Classic as of version 0.30.
 * In the goodie-bags of Minecon 2011, Diamond ore blocks, along with creeper, the player, and grass, were given out in the form of a foldable decoration.