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Very good advice here.

The ring is supposed to be a symbol of what you're proposing to her: marriage.

I'll give you the best rule of thumb for this, and you can close the thread:

By the time you're ready to propose, you'll know exactly what type of ring would look good on her, and she would like (based on her tastes, other jewelry, etc.) e.g. you wouldn't buy a giant band and giant rock for a girl that is tiny and wears tiny earrings, tiny rings, and so forth. You wouldn't buy yellow gold to a girl that wears mostly silver etc.

You buy a ring you're comfortable paying, and that you think looks good.

She'll be super happy because the ring is a symbol.

If she's not, dump her ass and run for the hills because she's not into you, she's into what your cash can buy.

Ciao