no idea on the drop rates them selves but I pretty much only get Highborn scrolls, though I am focusing on Night elf areas and maybe have completed 2-3 troll artifacts as compared to the 5 fossils and 11 Nelf ones
I think parts of it has to do at which dig site you are at. If you are at a Troll one you are more likely to find troll ones. Here's a map to each zone and what relics they can have.
Kalimdor
Eastern Kingdoms
Outland
Northrend
On the continent map it'll say the name of the dig site when you hover over the shovel. Use your thinker and say to yourself "Hmm, is that a troll, elf or dwarf sounding place?"
Hint: things saying fossil or bone are fossils
As far as I've seen, Keystone drop rates seem to plummet as your skill inceases. I've solved about 80 items and am currently at 525 skill - and it just seems terrible. The only keystones that drop with any steady pace are the Tol'Vir ones - aside from that, the rest are very, very low.