Leaving
rocks or stones after visiting someone's grave is a Jewish custom.
“...
Unique to Jewish tradition, however, is the custom of placing
pebbles, stones, and small rocks on Jewish graves. … the origin of
mourners leaving pebbles, stones or rocks at the site of Jewish
graves is unfortunately lost to time. Many theories exist, however,
...”
Read
more: Why Mourners Place Stones on Jewish Graves
The referenced article discusses eight (8) theories. As a student of the Bible, before
reading this article, I immediately thought about Theory No. 3: God
commanded Joshua to create a memorial in Jordan comprising 12 stones
that would represent the "children of Israel for ever." Check out the article to read the other seven (7) theories. It's very interesting.
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Related Post:
Do You Dare Go Dowsing for Graves?