After a month (not this one_ of trying to figure such numbers out I gave up. Obviously there has never been much urge to publish such data...
I tries to guess this rarity factor in two ways:
Once, taking the value of mining output into account and then looking at demand.
The first approach seems the most reasonable. Some closely related stats would be the evaluation of mining output, since that number has to take into account the relative volume and value of cuttable and industrial rough. My assumption was that extraction does not depend on distribution decisions (meaning, no one stops the mines if there is a slight dip in demand). Since further estimation (taking into account distribution decisions, as David explains) seemed to be very involved, this hobbyist gave up. If you really are very curious, you could try to trace this further.
But the second is more relevant, it seems.
Here's why I think so:
Anyone can track sales for a sample of diamonds using these online databases. meaning, you can make up a reasonable sample of, say D/IF and H/SI stones of a small range of weights (say, about 0.5, 1 and 1.5 cts) and see what sells first. Been there done that... and, it seems that 50 pointer D/IFs sell as fast as 1.5cts H/SIs.
Since I would not dare show the stats to my professor, I would not say where the experiment was conducted either. A more precise one could be easily mounted up to check this.
So... demand is crucial in making up this "rarity convention" (words taken form David’s post, again). Simply not too many would prefer these high grades at this price level. This is not surprising at all given pricing (as combinations of high color and clarity are more expensive than either high color or high clarity alone).
Meaning, it makes little difference what comes out of the ground. As long as the diamond market and pricing looks like today, my best guess is that allot of flawless rough will not make high grade diamonds less rare in jewelry.
Hope this makes some sense, despite the not so organized exposition.
This is about all I know about the matter.