That's simply the way she makes it sound. If it were extremely abundant, why would she only have the one? And yes, far-fetched maybe. But so is a large parasite from space falling to a planet and slowly devouring it over the period of 65,000,000+ years. I say Dreamstone falling to the Planet is just as reasonable as Lavos falling to it.
We know that Lavos falls to the Earth, so thats an invalid point. What your saying is that somehow, a piece of Lavos broke from his shell, and then accelerated so that it was faster than Lavos, so it got to Earth first. And that is far-fetched.
Listen, here is an idea thats much more feasible than your idea. Dreamstone is a naturally forming substance. Its not exclusive to earth, however. The same can be said of alot of minerals, so the idea is not that far-fetched. Its possible that Lavos's shell is made of Dreamstone. In Nature, we have oysters forming perals. Its feasible that Lavos secretes a substance that crystalizes into Dreamstone. After all, the Frozen Flame and Dreamstone do look alike, as you say over and over again. And if the Frozen Flame is a splinter of Lavos, and the Frozen Flame is made of Dreamstone, then thats only the next step in logic that his shell is made of dreamstone.
However, the Frozen Flame is a splinter of Lavos, and connects to Lavos. If all Dreamstone came from Lavos, as in, they are all splinters of Lavos, they would all have the same functions as the Dreamstone does. But they don't. So that strongly implies that not all Dreamstone is connected to Lavos.
In short, its absurd to think that Ayla's Dreamstone is all the Dreamstone there is. Hell, if you want further proof, look at the Masamune and Mammon Machine. Both are made of Dreamstone. Are you going to tell me that the materials used to make both of those came from that single rock Ayla had?