Like the phrase "marble town" is used to refer to a graveyard, 'marble house' is used to refer to a single grave. And like the term 'marble town' is used to comfort those who have lost a loved one, 'marble house' is an equally comforting euphemism. (Also, incidently, "Marble House" is a kickass song by Swedish band The Knife from their album 'Silent Shout').
"Don't worry Martha... James isn't really gone. He's just resting in his marble house. He's safe, Martha. I promise."