Some dogs will eat rocks because they sit under the deck behind the bar-b-que and they taste good.
It sounds though, like you have made rocks in to a toy, which is an *extremely* bad idea. Not only are they bad for her teeth, but they can accumulate in her gut, and cause serious problems (requiring surgical correction). If you don't already, I would seriously consider taking out pet insurance on this dog (after getting her checked by a vet).
Sounds like a GI blockage just *waiting* to happen.
In the meantime, stop throwing rocks for her, and train her to avoid the rocks, rather than play with them. If there is a chance the rocks might be tasty --- remove them from the area.
**edit --- Its also 'normal' for labs to get intestinal blockages. Seriously, when ever someone calls the vet clinic and says 'my dog ate ----', my first thought is - is your dog a lab?
I have two labs, and they are both wonderful, wonderful dogs, and neither of them have had any blockages, but I would guesstimate that > 75% of the calls to the clinic with a blocked dog are about labs.