What does the diamond Eric offers the narrator represent in Weike's, Chemistry: A Novel?

The diamond on an engagement ring supposedly represents the strength of an unbreakable bond, diamonds having been recognized as the hardest mineral throughout much of history; however, the narrator evokes the diamond as a symbol of inadequacy. Her immediate reaction to Eric's proposal is to note that according to a recent scientific article, lonsdaleite, not diamond, is the hardest known mineral. To the narrator, then, the diamond represents how the ideal of marriage falls short of the reality she knows.