What effect does the author create by comparing the silk stockings to a tiara of diamonds in "A Pair of Silk Stockings"?

Comparing the silk stockings to "a tiara of diamonds" implies that they are, indeed, a luxury and an item for the wealthy.

When Mrs. Sommers touches the pair of silk stockings, sensory words and images of touch and of sight are used to express the appeal that this item holds. In fact, the silk stockings seduce Mrs. Sommers as "[T]wo hectic blotches came suddenly into her pale cheeks," and she asks if there are any stockings in her size. Because the temptation to pamper herself becomes too great, Mrs. Sommers purchases a black pair of silk stockings.

Having succumbed to the temptation to indulge herself because she is "a little faint and tired," Mrs. Sommers satisfies her urge for freedom from her obligations:

How good was the touch of the raw silk to the flesh! She felt like lying back in the cushioned chair and reveling for a while in the luxury of it. She did for a little while. Then she replaced her shoes....[and] crossed straight over to the shoe department....

For the rest of the day, Mrs. Sommers frees herself from obligations, luxuriating in more purchases, a meal, during which she "wiggled her toes in the silk stockings," and a play in a theater. At the end of the day, "it was like a dream ended" as the cable car returns Mrs. Sommers to her obligations.