The diamond ring and the family car are both 20th century inventions. I thought the rule was spending 52 weeks income on a car and 4 weeks income on a diamond ring. Now it's 21 weeks for a suburban necessity and 13 weeks for a shiny decoration. What strange priorities for a not so brave, new world.
Having a car as a 20th Century invention is rooted in the undeniable utility of having a vehicle that can transport people and goods long distances quickly.
The "requirement" that every marriage requires a large investment up-front in a diamond is purely the product of a marketing campaign by a diamond cartel (i.e. a price-fixing monopoly). Here is a brief, but honest as far as it goes, account by an organization dedicated to selling you diamonds. Note that the founding of the American Gem Society, in 1934, precisely coincides with the start of the De Beers cartel hard sell. It is striking that a trade organization, founded as part of a marketing scheme, does not try to hide its own origin.
Lengthier accounts of this bizarre pure luxury business are easy to find.
Regarding those "guidelines" (treated as "rules" by the industry) for how much to spend in a diamond ring - if you let marketing literature tell you what you should spend money on, you are perhaps typical, but still you are just being a patsy.
When I got married, no diamond was involved. It was a ridiculous waste of money, absolutely not an investment (they are virtually impossible to resell, the industry makes sure of that), and at that time a prop for the Apartheid regime. That last reason had ended, and recent "conflict diamond" rules have greatly reduced the tendency of diamond purchases to support mass murderers, but not ended that last problem entirely.