You have to be in Tel Mur, don't have to be a Gargoyle, nor do you have to be at GM. My elf lumber jack got one within five minutes, but then I didn't get another after at least an hour or more of lumber jacking non-stop. I'm guessing you might get more chances at GM level, which I'm now at, but haven't been back to try (still looking for the illusive rare wood trees that you can only get at GM). In general, I've found that there's a boat load you don't get at all until you hit GM lumber jacking. It'd be interesting to get one of those +5 hatchets to see how that effects chances at rare resources.
By the way, I ran accross a minor (not a gargoyle) whose only reason at all for mining in TelMur was for blackrock. He was GM, elf and said he was getting a fair number of them. So they probalby work the same as crystalls and such.