Both of these excellent short stories concern themselves with the theme of materialism, and in particular its dangers. In "The Rocking-Horse Winner," for example, it is Paul's mother who is consumed with the desire for more money, and this longing is something that impacts not only Paul but the very house where they live:
And so the house came to be haunted by the unspoken phrase: There must be more money! There must be more money! The children could hear it all the time, though nobody ever said it aloud.
It is this desire for more wealth that affects Paul so strongly, and eventually, results in his death, as he weakens himself progressively further through a supernatural relationship with the rocking horse that allows him to predict which horses will win. As Uncle Oscar comments at the end, although Paul's mother may have won lots of money, she only won that money at the cost of her son's life, questioning the value of wealth compared to other things.
In "The Diamond as Big as the Ritz," Fitzgerald likewise places materialism under the microscope through writing a kind of pseudo-farce. Fitzgerald uses his typical strategy of employing a protagonist who emerges from a humble background and moves to more exalted territory in terms of his education. Percy Washington, the friend that John makes at his new, exclusive school, is shown to come from a family who prize wealth so highly that they are willing to commit any crime, including murder and retaining slavery, in order to keep it safe and retain it. The ending, which is obviously ironic, shows that John will have to return to his humble hometown, which is humorously called Hades. Of course, Fitzgerald has shown that the real "Hades" is actually the Washington's establishment due to the infringement of human rights and laws there. John's inability to see this shows he has missed the point of the story: wealth corrrupts.