If each item has an equal likelihood of appearing (which it should) and each outcome is independent of one another, then based purely on items:

(9 stones/26 possible items (gems+stones)) = 34.6%

Odds of obtaining a specific item: 1/26 = 3.846%

However, you also have a chance of encountering Drilbur and Excadrill. I don't know the chances, but if we treat it as an outcome that is just as likely as an item (I slightly doubt this is the case) then:

Chance of obtaining a stone: 9/27 = 33.33%

Chance of obtaining a specific item: 1/27 = 3.7%

I'm inclined to say that encounter of Drilbur is between 30 and 50%, but I'll just leave it as 3.7% as it'll just drop the stone chance by 3 to 11% and specific item chance by 1% if I did calculations correctly.

- From the First Link