Symbolically, the silver is "spongy" due to its power to absorb the differing dreams of every protagonist. Simultaneous with the first touch upon the silver we get one of the clearest statements of the novel's theme of relativism, in which facts have meaning only as subjectively interpreted by man.
"The hazards of the world" refers to precisely this profusion of interpretations, while the "dark depths of the Gould Concession" refers not only to the physical depths of the mine, and the emotional depths of Mr Gould senior's suffering, which inspired the mine, but to the more sinister, abstract "depths" awaiting anyone who surrenders to the claim of an overmastering dream-ideal.
Note that although the narrator takes pains to remind us that Mrs Gould is
"unmercenary," the passage makes clear that Mrs Gould, equally with
Charles Gould and Nostromo, is a character enslaved by the silver, to the exact
extent of her idealism.