Stones are set around bench and stairway with mortar set behind them so it is not visible.
I like using native stone. Here on Long Island, we are blessed with a fine blend of every stone found between here and the arctic, thanks to the last few glaciers; and for the same reason, they are generally rounded. I lay them up so that the mortar is generally invisible, not that you can't see them if you peer in between the rocks, but at first glance they appear to be dry stacked. For paving purposes, I cut the stone to fit together.
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