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, Hoes, Swords, and Armor of the highest quality. It is also a requisite ingredient in Jukeboxes, and solid diamond blocks.

Prospecting
Diamond Ore can be found in veins of 1-9 blocks (with larger concentrations being more rare). Diamond shows up at elevations ranging from 1 to 20 blocks above the bottom of the map.
 * Below elevation 18, it occurs in roughly 0.09% of rock.
 * This calculates to approximately 4 diamond ore per chunk.
 * Mining 771 blocks below elevation 18 has a 50% chance of finding at least one diamond ore.
 * There is never more than one vein of diamond ore per chunk. However, the area in which the vein can be located is offset from its chunk by +8 on both the X and Z axes. So, each vein has its own 16x16 area centered on the intersection of four chunks. This principle can be exploited to make mining more efficient i.e. whenever you find a diamond vein, start searching in a different 16x16 area.

Trivia

 * When diamond was first introduced, it was called emerald.
 * If diamond ore is obtained via inventory hacking, 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.
 * Like other ores, diamond was about 50% more common in the quadrant south-west of the point (0,0) compared to the north-east quadrant. However, this was fixed in Beta 1.6.
 * 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, but, due to a bug in 1.6 (and only 1.6), Clay was the rarest material in the game.
 * Diamond ore cannot be found or placed in Classic as of version 0.30.