It is illegal to take items from the parks, even parts of living plants, and I guess even rocks.  It is also illegal to dig for artifacts.  
When I look I scrutinize any bare spots of the campsite, especially around where the camp fire is or likely was in the past.  Seldom do I find anything larger than tiny chips of stone, from which you can get an idea of the kind of rock was being used.  A lot of chips probably mean a lot of past activity on the site.
I just toss mine back onto the campsite.  Most people either do not look or do not see them, those who do will be able to enjoy the finds as I initially did.
I happen to live on a point on Lake Vermilion, at the southern edge of the BWCA, which has apparently been a campsite for thousands of years.  We regularly find artifacts when digging in the vegetable garden on the point.  I've attached a couple photos of our finds.  With a treasure trove like this here, I do not have to go far, or take anything from the parks, to enjoy these items from the past.
Re: Photos are of arrowheads and pottery shards (c 1000 AD) and a few fur trade artifacts from the late 1700's to mid 1800's.  We found an 1863 nickle and also recently an 1897 silver dollar, the serpent side plate off a trade rifle, metal awls and buttons, musket balls.

:o

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