From analogy with an orchard, because the marble or other stone grave markers are arranged in rows like planted trees.
marble orchard (plural marble orchards)
- (slang) A cemetery.
1927, Mary Roberts Rinehart, Lost Ecstasy: A Novel, page 258:
- "It's so peaceful." ¶ "So's a marble orchard." ¶ "A marble orchard?" ¶ "A cemetery."