I watched a jet after take off from Billings MT on a sunny cloudless day go over Yellowstone. I live one mile from the Eastern side of the Rockies. As I watched the contrails, (I'm a pilot) something came out and moved north (the jet was going west) along the Rockies, It was a bluish gray slightly darker than the sky. I watched for several minutes, bright white crystals were blinding me as I watched for several minutes, then it disappeared. There was a small cloud where I could no longer see it travel. The crystals immediately disappeared as well. At first I assumed it was blue ice, but it was as large as a leer jet, at least. It was over the local airport that allows small aircraft only. I watch planes land there everyday but have never seen one that was camouflaged with the sky before. I am a senior and have never seen a UFO in my life.