Most of the big diamonds form deep in the Earth, between 410km and 660km (250 to 400 miles deep), and nearly all of them are older than 1 billion years.  This is one reason why they are so interesting, because the mineral inclusions stuck inside these are essentially windows into Earth's geologic past and also a glance of what the deep Earth chemistry is like.  Diamonds are the only way we can get samples of these inaccessible locations, but are critical for our understanding of Earth processes.  Diamond research as been use to as evidence of the onset of plate tectonics, crustal recycling, as geobarometers and geothermometers, and as a source to discover new minerals that only grow in these parts of the Earth.