Pretty sure gem nodes are random. If you find a big moonstone, you can mine around and likely find more big moonstones. Even if you mine different rock types. Not sure if level matters, I don't think it does.
Coincidentally, I'm stuck on "Mine a moonstone" in a Gem Hunter credo. I'd assume gems are more common on deeper levels but have no evidence of it. They're definitely determined by nodes, like DDDsDD999 described.