When the Prince of Morocco chooses the gold casket, he finds a scroll inscribed with the idiom,
All that glisters is not gold.
In other words, just because something glistens beautifully doesn't mean it's as lovely as it appears on the surface. Meant as a warning against greed, this sentiment seems strange coming from Portia's wealthy household in prestigious Belmont. This dichotomy echos the tension between love and ownership in Merchant. Another, similar source of tension arises as Bassanio pursues Portia. Through the idiom, Portia's father had aimed to protect her from marrying a greedy man (one who "glisters"), and while Bassanio does seem to love Portia as a person, he nevertheless proves greedy and certainly values Portia at least partially for her wealth.
The idiom implicates broader tensions as well. Many characters who seem to "glister" prove morally flawed—from well-regarded yet selectively hateful Antonio to Jessica, who heartlessly sells her mother's ring for a monkey. Nevertheless, these people and their values maintain power in the play: the Christians emerge victorious and can continue to exact cruelty on those unlike themselves. So while all that glisters may not be gold, the society depicted in the play doesn't seem to care.
