Conspicuous consumption became consumer capitalism, where people started to construct their identities piecemeal from the goods and services they consumed rather than derived ground-up based on their place in either an industrial or pre-industrial hierarchy.
The thing about a consumer identity is that it doesn’t require the consumption to actually be visible to others, it just requires the self to be accreted through the consumption. So far, so standard theory. I personally would suggest on top of this, however, that the concept of “conspicuous consumption” can be extended within this framework to encompass a “conspicuousness to the self”. In other words, goods (or services) bought to reinforce the self-concept, to show-off within the Eigenweld that one is successful or worthy. An engagement ring strongly fits this extension of conspicuous consumption.