Joshua,
I have verified with GIA that they do in fact identify rough diamonds, the price is based on carat weight. My thought is having a 25 carat stone with hundreds of thousands of dollars and your concern is it would cost a couple of thousand dollars. It just does not make sense. Out of curiosity what county did you find this in Idaho? Was it alluvial or in kimberlite?
It would seem to me that there is an avoidance to verify the stone as diamond. I guess my other confusion is why you are posting this on here for evaluation when that is virtually impossible.
Anyhow I wish you the best.