Explain the symbolism of the simile "...at intervals she appeared suddenly at his side like an angry diamond and hissed: 'You promised!' into his...

In Ch.3 Nick Carraway attends a party at Jay Gatsby's house. Initially, as soon as he arrives at the house he is completely overwhelmed by the lavish splendor and extravagance of the party:

On buffet tables, garnished with glistening hors-d'oeuvre, spiced baked hams crowded against salads of harlequin designs and pastry pigs and turkeys bewitched to a dark gold.

A little later as the party warms up, he views the anonymous interpersonal interactions of the party guests very cynically:

The bar is in full swing, and floating rounds of cocktails permeate the garden outside, until the air is alive with chatter and laughter, and casual innuendo and introductions forgotten on the spot, and enthusiastic meetings between women who never knew each other's names.

At about two in the next day Nick notices that the men are reluctant to return home with their wives. They wish to continue flirting with the other beautiful women:

I looked around. Most of the remaining women were now having fights with men said to be their husbands. Even Jordan's party, the quartet from East Egg, were rent asunder by dissension. One of the men was talking with curious intensity to a young actress, and his wife, after attempting to laugh at the situation in a dignified and indifferent way, broke down entirely and resorted to flank attacks--at intervals she appeared suddenly at his side like an angry diamond, and hissed: "You promised!" into his ear.

The wife's anger and rage are expressed in her eyes which glint and sparkle like a diamond and she hisses her displeasure to her husband like a snake saying that he promised to behave and that he has been promising her perhaps for a very long time to take her home.

One of the most important characteristics of modern fiction is that the prose is poetic. Here Fitzgerald very poetically uses a simile to describe the eyes of the wife glinting suggestively with rage to that of a sparkling diamond. At the same time the simile itself is an instance of 'pathetic fallacy' for the inanimate diamond is endowed with the human quality of anger.

Ruskin who coined the term 'Pathetic Fallacy' in his "Modern Painters" (1856) defined it as “to signify any description of inanimate natural objects that ascribes to them human capabilities, sensations, and emotions."