It is wrong to wear diamonds before dusk, except on one’s marriage rings. Before, after, and during breakfast, luncheon and dinner, it is vulgar to wear a mixture of colored precious stones. It is always a comfort to know that so many things one can’t afford to do anyway are vulgar.

Judith Martin (b. 1938) American author, journalist, etiquette expert [a.k.a. Miss Manners]

“Miss Manners,” syndicated column (1980-12-28)
    (Source)

Reprinted in Miss Manners’ Guide to Excruciatingly Correct Behavior, "Answers to Questions Nobody Asked" (1983).