Diamond

Diamonds are vanilla items, dropped when Diamond Ore is mined. Diamonds can also be obtained through Macerating Nether Diamond Ore. Doing this will give you 2 diamonds, as opposed to smelting it, which will only give one.

Industrial Diamond
In Tekkit, Diamonds can be created using Compressed Coal Balls. More about this process can be found here.

Uses
Philosophers Stone

Diamond Block

Diamond Drill

Diamond Pipe

Energy Crystal

Glass Fibre Cable

Or when you have RedPower2 installed

World Anchor

Diamond Handsaw