I too would suggest that you stick with what you can get at the fish store. While boiling will kill any germs on the rocks, the rocks themselves can leach stuff into the water.
I tried washing and soaking some pea gravel. After cleaning it real good, I let it soak in a bucket over night. The next morning, the water had a brown discoloration (this is just fresh water from the tap with the rocks in a bucket... NOT in the fish tank). I changed the water, washed the rocks again and soaked them for another day. Next morning, water was still rusty brown from something leaching from the rocks. I continued this routine for a week before I finally decided it was SOO much easier to just spend $5-10 for rocks from the fish store. They still had to be washed to get excess dust off of them, but at least after soaking over night, they did not change the color of the water.