The "slight" problem with that is that clusters are discontinuous between Z-levels. After revealing, clusters really look like oval-ish blots all over the map, randomly positioned on each Z level. Other then that, and the fact that purple stone would kinda freak me out, I don't have all that much against kimberlite representing as clusters. I'll need to lower its chance of appearance though (to some 10%), it's waaaaaay too common otherwise.
Edit: having read some more on kimberlite on the wiki, it seems a kimberlite pipe should be rare (so even 10% might be a bit too much, but we've got a fantasy world here), relatively large, and quite possibly not contain any (gem quality) diamonds at all. An idea deriving from this is to make a True Diamonds mod in hopes of it being picked up by Toady at some point.
The concept:
Make kimberlite spawn as large clusters, and far less frequently, thus making it more realistic, and fixing it in the process.
Make "non-gem-quality" diamonds, labeled simply "tiny diamonds" a lot more frequent in kimberlite, and reduce the chances of normal-size diamonds appearing a bit.
Also, up the value of all the diamonds for their rarity.
Add various "complementing" minerals to kimberlite in small clusters also, but it'll have to be tested more, because it may disrupt the spawning of diamonds themselves, and isn't altogether relevant to gameplay.
I also wonder, since kimberlite is an intrusive igneous rock type, if it can be made into a rarely occuring layer of sorts, but that would be Highly experimental.
[ November 24, 2007: Message edited by: Sean Mirrsen ]