Actually - I like the tattoo concept. In order for it to be effective, it's not that it has to "Represent a significant economic expenditure" - rather, it has to be something that makes it infeasible for the proposer to do many times. A large economic (for the proposer) commitment is one way of doing this - but tattoos (or better yet, branding/scarring) would be even better.

With regards to the problems with diamonds. Totally agree, I think they are absolutely stupid. In a reasonable/responsible world, the proposer would have to demonstrate the ability to provide material food/water/energy/shelter security. Now that, has "real" value to me.

But, I'm clearly not the target for an engagement ring.