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, Gold and Diamond Pickaxe.

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

Prospecting
Diamond Ore can be found in veins of 1-8 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 1.276% 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.
 * If diamond ore is obtained via inventory hacking or the silk touch enchantment, smelting it will give the diamond gem.
 * 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.
 * Diamond ore cannot be found or placed in Classic as of version 0.30.
 * As of Beta 1.9 Pre-release 4, this block, along with any other blocks which drop something else when mined, can be obtained legitimately in inventory by using a tool with the enchantment 'silk touch'.