"The Diamond as Big as the Ritz" may be a fun fantasy story, but it's also a major critique of American history and American values. One of the early tip-offs is Percy's explanation that his family descends from George Washington and Lord Baltimore – two men who were integral in the founding and expansion of our country.
The story of Fitz-Norman Washington, Percy's grandfather, quickly becomes a parallel for the expansion of the U.S. into the west. Fitz-Norman set out after the Civil War to seek his fortune; when he found that fortune, he exploited the country's natural resources for his own material gain and then safeguarded that secret through the manipulation and pain of others. The slaves are a key example here. Fitz-Norman took advantage of them by convincing them that the South won the civil war and that slavery was still legal. He then convinced them his giant diamond was a rhinestone mine, and kept all the profits for himself.
Fitzgerald makes the point that material success has its costs – and that those who seek it blindly falsely believe that the exploitation of others is natural for their own purposes. A great example is the passage in which we learn that Fitz-Norman "was compelled, due to a series of unfortunate complications, to murder his brother, whose unfortunate habit of drinking himself into an indiscreet stupor had several times endangered their safety. But very few other murders stained these happy years of progress and expansion," (4.11). Through this hyperbole, Fitzgerald points out how absurd it is to sacrifice human life in the name of material gain.
The giant diamond itself is a symbol in this overarching satire. To begin, it is an emblem of the garish excess of the Washingtons' wealth. Excessively large diamonds are considered vulgar; so a diamond as big as the Ritz is the epitome of tacky glut. It's also significant that Washington built his château on top of the diamond – he's built his home, literally, on the mountain of his wealth.