How did Rosamond persuade her mother to buy the things she saw in the shops?

The phrase "all that glitters is not gold" in 'The Three Caskets' signifies that appearances can be deceptive, and true value or worth is not always evident in outward show or superficial attractiveness.

This proverb serves as a core thematic message, warning characters and readers against making judgments based solely on outward display. It underscores the idea that what seems appealing or valuable on the surface might, in reality, be worthless or even harmful, while something seemingly plain or undesirable could hold profound truth or beauty.

In the story, this concept is vividly demonstrated through the three caskets presented to Portia's suitors. The gold and silver caskets, which are outwardly grand and appealing, are chosen by the Prince of Morocco and the Prince of Aragon respectively, both of whom fail. They are deceived by the dazzling exterior, only to find them filled with grim or mocking contents. Conversely, the plain, unadorned lead casket, which held little visual appeal, is the one containing Portia's portrait, symbolizing true love and worth, chosen by Bassanio who looked beyond superficiality. Thus, the phrase embodies the story's lesson that genuine substance often resides beneath an unpretentious exterior.