[grw_]

For houses it's completely plausible that the motivation is for the asset to appreciate in value and even if the marriage ends in divorce, both parties end up being able to extract some value from it. For a retail diamond the purchaser is likely to see zero value recovered from it whether divorce happens or not.

For cars, I suppose my hypothesis would say cars likely to depreciate very quickly (such as high-end SUV) are more suitable as engagement gifts than practical (prius or such), which fits roughly with my observations in real world