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Is this a European tradition?
It pretty much appears to be, there are also the coin trees where people pound coins into logs or trees. Most of the references I find to those are in the UK
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No, it's a weird Internet tradition, not entirely unlike those "love locks" people attach to fences, bridges etc.
Also European, mainly in France.
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I've got passers-by leaving a small rock on top of gravestones in the churchyard my wife and I take care of.
I expect that's also something from the internet age.
No it is an ancient tradition, especially among those of the Jewish faith. It is a symbol meaning "I was here and I care." See the end of "Schindler's List".
Coins left on headstones could be a American tradition especially among the military. Leaving a cent means someone was there, a nickel means they trained together, a dime that they served together, an quarter that the visitor was with them when they died.