I've seen this done in Catholic cemetaries and in some modern Native American cemetaries. As for the rocks and crystals, I'm not sure of the symbolism. I think the little trinkets left behind are things that represent the deceased's loved ones interests they had in life. For example, I've seen little matchbox cars and trucks on the grave of a young man who was a mechanic. I've seen dolls on the graves of little girls. I think a lot of it is to give the families a little comfort that their loved one is enjoying the things they loved in the afterlife.
I always put a small Christmas tree on my father's grave for the holidays. He always enjoyed the Christmas season, so for me, it is symbolic. The last time I took my nephew to visit his grandfather's grave, he left his yo-yo so "Grandpa could have something to play with."
Hope this helps a little!
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