I'm really surprised no one has asked me about this yet, but nonetheless I have endeavored to provide an answer. It's pretty difficult to build up a stash of trillium ore, over the past month I've only managed to get a few stacks of it. Meanwhile I've probably collected over 1000 ghost iron ore. But, I saved what little I found in case I needed it to level professions At this point I haven't used it - so I thought I'd do a bit of testing with prospecting.
I only had 2 stacks of black trillium and one stack of white. They seem to have similar, if not the same, prospecting tables. Here's what came out of them:
3 stacks of trillium ore:
14 uncommon gems (3 purple, 2 yellow, 2 red, 4 orange, 2 blue, 1 green)
13 rare gems (4 purple, 4 yellow, 1 red, 4 orange)
21 sparkling shards (2.1 serpent's eye)
It seems pretty clear from this sample that the drop rate for rare gems is much higher than for ghost iron ore. Unfortunately this isn't a great sample size in terms of statistical analysis, but I hope it's enough to give you number crunchers an idea of what trillium will be like.
Feel free to leave questions/comments below!