Zenster: well, I checked the net for references, and this is what I came up with, in order of increasing detail:
from http://volcano.und.nodak.edu/vwdocs/minerals/diamonds.html
Kimberlite magmas are rich in carbon dioxide and water which brings the magma quickly and violently to the surface.
from http://geology.about.com/science/geology/library/weekly/aa021598.htm
You know that a diamond is a hard, dense form of pure carbon. Physically there is nothing harder, but chemically speaking, diamonds are pretty fragile. More precisely, diamond is a metastable mineral at surface conditions. Experiment shows us that it cannot form except under conditions found hundreds of kilometers deep in the mantle beneath ancient continents. Take them a little above those depths, and diamonds swiftly turn to graphite. At the surface they can endure in our gentle environment, but not anywhere between here and their deep birthplace.
Well, the reason we have diamonds is that they cross that distance quickly, in just a day or so, in very peculiar eruptions. Aside from impacts from outer space, these eruptions are probably the most unexpected occurrences on Earth. Have you seen footage, or just a cartoon, of an oil gusher? That’s how these work. Certain magmas at extreme depths find an opening and rush upward, burrowing through various rocks-including diamond-bearing zones-as they go. Carbon dioxide gas comes out of solution as the magma rises, exactly like soda fizzing, and when the magma finishes puncturing the crust, it explodes into the air at several hundred meters per second.
We’ve never witnessed a “diamond eruption,” although one happened in Antarctica just a hundred thousand years ago or so. Geologically speaking, that’s just last week. But they have been very rare since about a billion years ago.
from http://vishnu.glg.nau.edu/cev/barbara.html
Nature of the erupting magmas: Kimberlite magmas have a relatively rapid journey from the mantle to the surface that allows them to carry, and preserve, large volumes of xenolithic mantle material,
including diamond.
(snip)
During the last ~3 km of its journey to surface, the magma does not rise rapidly, and, prior to final breakthrough, the volatile-rich kimberlite behaves essentially as a closed system.
(snip)
Explosive breakthrough occurs when the volatile pressure exceeds the confining pressure. A crater is excavated.
The last of these goes into much more detail than this, check it out. So to sum up, diamonds are quickly brought up from the mantle to just below the surface, slow down for most of the remaining few km, and finally explosively erupt to the surface.
Bill