This is one of the novel's most evocative lines describing the alienation of the Goulds from each other, and the dehumanizing coldness of the silver.

The full import of these lines involves the symbolism of the silver as the dream-ideal, emphasized here with the line "it had been an idea." Originally, the ideal of San Tomé, and by extension all ideals, appeared as a unifying force -- uniting not only the Goulds in their idealistic marriage but the nation behind the success of the mine. By now, however, it has become apparent that the true nature of the dream-ideal is to possess, isolate, and separate its victims, resulting in disunity. The "wall of silver" image is not meant to refer merely to the lovelessness of the material interests (though that is there too) but to the nature of all dream-ideals.

In that context, it's interesting to note that the line contains a grammatical confusion: one can read it as saying that either the inspiration, or Emily's heart, turns into the silver wall.