http://www.wowhead.com/?item=23424#V0Rzc and http://www.wowhead.com/?item=23425#m0Rzc seem to be useful numbers.

The wowhead prospecting numbers are a little confusing. My experience, after ~50 stacks of adamantite prospects, is that I have about a 66% chance of getting a blue gem from a stack of ore. Anytime I prospect 3 stacks, I fully expect two blue gems. So the wowhead numbers would indicate that a single prospect of 5 gems has a 2.5% chance to drop a given gem, or a 15% chance to drop any blue. A stack of adamantite is 4 prospects, so you'd have a 60% chance at a blue gem. That's close enough to my experience that I'd agree with it, but don't be deceived by the 2.5%.

So far it's profitable for me to buy and prospect adamantite below 25g a stack since I can cut all the gems I receive and sell them for more than an uncut gem. Mileage will vary depending on your actual selling price.

I've also noticed that there now no good designs on the AH. Once in a while I'll see a good one, but it's something I already have. The total quantity is definitely down compared to a few weeks ago when more people were actively killing mobs to level. I would feel sorry for people trying to level jewelcrafting right now, but the greed overcomes the pity.

Your own numbers are a little confusing too, you use percentages for stacks (4 prospects) in the beginning and then switch to single prospects numbers.
But the last couple of weeks I've been getting 1-2 blue gems per 10 prospects on a regular basis, which matches your 15% number quite well and I've prospected maybe 80-100 stacks of adamantite now.

Sometimes you get a lucky prospecting streak though

I bought the AH clean of adamantite, and those numbers look right to me. Trouble is, my server's green gem economy seems slow - sure, they sell for 3-5g each when cut, but you can only sell 2-3 of each cut per day, and only 2-3 cuts per colour are profitable.

Demand for the greens is just too low, even though there's enough cash floating round the economy that the few people who do want them don't mind paying well over vendor price. Normally, where supply >> demand, you'd expct the undercutting to take it right down to vendor price - doesn't seem to be happening for some reason.

The upshot is that I've made a paper profit of almost 1k gold, but it'll take me a month to realise it. And you can't afford to lose your deposit more than once on a green gem, since that'll eat your entire profit.

I think Blizzard should let you combine 20 green quality gems into 1 blue quality gem. Price-wise it'd be about right, and it'd mean you don't get swamped with greens that just won't sell.