Are these River Stones Safe for Aquarium?

I just purchased several bags of river stones made by Decor Stones - AKASHA River Stones from Walmart. I always boil my stones before placing them in the aquarium. I was a bit concerned when I boiled the stones because the water was allot darker than other stones I have boiled and the pan had a waxy feel around the water line. Does any one know if the stones real (nature made) river stones or are they man made material?