I'd rather buy her a car, even if it is 2x, 3x more expensive than a ring. At least it has some utility and it definitely has a wow factor to impress her friends with.
If this kind of gift is made to be symbolic of your relationship, then a car (or any other heavily depreciating asset) isn't the best symbol.
One of my friends got an olive tree for her 25th wedding anniversary. I thought that was a great gift.
And diamonds are a heavily depreciating asset. A car loses about 10% of its value the second you sign the purchase contract. With a diamond ring, it's more like 60%.
"Look honey, you could stand on tradition and insist on an overly ostentatious ring... or I can get you a Lexus. Up to you!"
"Oh that ring's lovely, did you see my wedding gift? It's parked just outside."
/s