Young John T. Unger, who had just turned sixteen, had danced all the latest dances from New York before he put on long trousers. And now, for a certain time, he was to be away from home. That respect for a New England education which is the bane of all provincial places, which drains them yearly of their most promising young men, had seized upon his parents. Nothing would suit them but that he should go to St. Midas' School near Boston—Hades was too small to hold their darling and gifted son. (1.2)
Age is an important part of John's character – we join him just as he turns sixteen, and so we expect that this story will be as much about the tribulations of youth as anything else.
John T. Unger was on the eve of departure. Mrs. Unger, with maternal fatuity, packed his trunks full of linen suits and electric fans, and Mr. Unger presented his son with an asbestos pocket-book stuffed with money. (1.4)
This opening suggests a coming-of-age tale. It's important that John is leaving home at the start of the text. His parents pack him off with supplies, and we expect that his journey will be metaphorical as well as literal.
"That's nothing." Percy had leaned forward and dropped his voice to a low whisper. "That's nothing at all. My father has a diamond bigger than the Ritz-Carlton Hotel." (1.25)
At first, it's hard to take Percy seriously – and this is in part because we remember that he's a teenager trying to impress his friend. Age plays a big part on John's character, too, but in Percy's as well.
John T. Unger
Afterward John remembered that first night as a daze of many colors, of quick sensory impressions, of music soft as a voice in love, and of the beauty of things, lights and shadows, and motions and faces. There was a white-haired man who stood drinking a many-hued cordial from a crystal thimble set on a golden stem. There was a girl with a flowery face, dressed like Titania with braided sapphires in her hair. There was a room where the solid, soft gold of the walls yielded to the pressure of his hand, and a room that was like a platonic conception of the ultimate prism—ceiling, floor, and all, it was lined with an unbroken mass of diamonds, diamonds of every size and shape, until, lit with tall violet lamps in the corners, it dazzled the eyes with a whiteness that could be compared only with itself, beyond human wish or dream. (2.30)
John looks at the château with a childlike wonder. There is something mythological and ancient about the scene, as emphasized with allusions to Ancient Greek mythology (or mythical characters borrowed from Shakespeare).
John lay quietly as his pajamas were removed—he was amused and delighted; he expected to be lifted like a child by this black Gargantua who was tending him, but nothing of the sort happened; instead he felt the bed tilt up slowly on its side—he began to roll, startled at first, in the direction of the wall, but when he reached the wall its drapery gave way, and sliding two yards farther down a fleecy incline he plumped gently into water the same temperature as his body. (3.4)
John is reduced to a childlike helplessness during his stay at the château. Not that he doesn't enjoy it. But as far as his coming-of-age, this isn't exactly helping him on his way to adulthood.
[Kismine:] "I'm very innocent and girlish. I never smoke, or drink, or read anything except poetry. I know scarcely any mathematics or chemistry. I dress very simply—in fact, I scarcely dress at all. I think sophisticated is the last thing you can say about me. I believe that girls ought to enjoy their youths in a wholesome way." (5.25)
If John is our glimpse into adolescent boyhood, then Kismine is the female equivalent. What picture does Fitzgerald paint of female adolescence? Is Kismine a fair representation? A caricature?
[John] was enjoying himself as much as he was able. It is youth's felicity as well as its insufficiency that it can never live in the present, but must always be measuring up the day against its own radiantly imagined future—flowers and gold, girls and stars, they are only prefigurations and prophecies of that incomparable, unattainable young dream. (5.3)
This is an odd passage, and along with the few final paragraphs of the story, brings the theme of "Youth" to the forefront. Might John's time at the Washington estate also be an allegory for the dream-like state of youth?
[John] was critical about women. A single defect—a thick ankle, a hoarse voice, a glass eye—was enough to make him utterly indifferent. And here for the first time in his life he was beside a girl who seemed to him the incarnation of physical perfection. (5.11)
It's probably no coincidence that Kismine – like her father's giant diamond – appears to be flawless. Check out her "Character Analysis" for more details.
John T. Unger
"You have made an enormous impression on me," said John's eyes, "and I'm not so slow myself"—"How do you do?" said his voice. "I hope you're better this morning."—"You darling," added his eyes tremulously. (5.9)
"Under the stars," [Kismine] repeated. "I never noticed the stars before. I always thought of them as great big diamonds that belonged to some one. Now they frighten me. They make me feel that it was all a dream, all my youth."
"It was a dream," said John quietly. "Everybody's youth is a dream, a form of chemical madness." (11.27-28)
Again it is hinted to us that "The Diamond as Big as the Ritz" is as much an allegory of youth as it is of wealth or religion or of America's history. "Chemical madness" is indeed an apt description of the sensory overload John experienced at the Washington estate, which is not dissimilar from the flush of first love.