Java Edition:Diamond Ore (Item)
Diamond ore was added in Indev 0.31 20100129 but the item form was not normally obtainable until Beta 1.9 Prerelease 4 with the introduction of Silk Touch. Diamond ore as an item could be obtained before being recontinued, only obtainable once in Beta 1.6 Test Build 3. This item was again discontinued for only 1 version, 12w32a, in which Silk Touch doesn't work.
Obtaining
Beta 1.6 Test Build 3 Debug Chests
In Beta 1.6 Test Build 3, any newly generated world would contain up to 11 debug chests with every item in the game. The diamond ore item was obtainable in the chests 2nd and 6th from the left in the 25th and 9th slots respectively, in stacks of 64.
Silk Touch
From Beta 1.9 Prerelease 4, Silk Touch was introduced allowing players to get blocks they couldn't get before. All a player has to do to obtain the item form is to mine a diamond ore block using a Silk Touch pickaxe. This doesn't work in 12w32a due to a bug.