As I was searching for some more inspiration for rocks and hoping to find some fun and interesting ways to integrate rocks and natural resources I came across this. I love this idea and tons of learning could come out of this.
And this could be another way of integrating rocks into other subjects! And playgound rocks are free. You could always have a hunt for smooth rocks and get your collection up and running for these alpha rocks pretty darn quick. If you look in the craft section of Dollar Tree you can find bags of good river rocks for a dollar!
Have fun investigating!