i would presume that the scoff came more from the implied "I am unable to understand or appreciate the needs and desires of a female partner" than it had to do with the logic behind the statement.

if you find a partner who doesn't want a diamond, then you are completely justified in not getting her one (just because she says she doesn't want one, doesn't necessarily mean she doesn't, though).

otherwise, the real value isn't in its intrinsic value, but in the various byproducts -- the signal it sends, how she's able to show it off to her friends (and will be expected to), the fact that she's likely been dreaming about it since childhood, etc.

just like any social norm, it doesn't always make sense. but by refusing to do something because you think it doesn't make sense, the real thing you're saying is that it's about your needs, not the other person's.