If it helps, I can explain why diamonds are typically only found in kimberlite pipes in the real world:
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Aside from a meteor impact (which will only form microscopic diamonds), the only place with the right combination of temperature and pressure needed to form diamonds is near the contact between the mantle and the thick crust of a continental plate.
Diamonds can't arrive at the surface by being carried up as a xenolith in magma, because the combination of high temperatures and rapidly decreasing pressure will destroy them relatively quickly.
Instead, diamonds encrusting the bottom of a cooling continental plate are occasionally brought up from depth (along with an wide asortment of other rocks) relatively quickly (for a geologic event) to form pipes of kimberlite. I don't think it is known for certain why these erruptions of rock occur, although there are a variety of theories.
In any case, kimberlite pipes are rather rare, and typically very hard to locate. To make matters worse, almost all kimberlite pipes contain no diamonds whatsoever, and even the very diamond rich ones contain diamonds in an incredibly small ratio to the rest of the kimberlite rock's mass.
Diamonds have also been found occasionally on river banks and the like; they likely come from eroded kimberlite pipes. The number of diamonds found in this manner is incredibly small when compared to the amount mined, however.
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So basically, it isn't at all unreasonable that you haven't found any diamonds, from a realism point of view. I don't know enough about what the raw's frequency data means in terms of how much gets placed to say if the game is working as intended or not, however.
I don't see any reference to it in the raw data, but I might as well throw this out there anyways: kimberlite pipes that reach the surface tend to weather faster than the surrounding rock, producing rounded pits inside which ponds frequently form; a good number of kimberlite pipes have been found by looking for unusually rounded ponds.
I don't think Toady went so far as to model that feature of kimberlite pipes, though. (At least not yet.)