Yes, there is cubic zirconia and things like that. But notice how society reacts, and rejects cubic zirconia? It isn't because cubic zirconia is inferior. It is because it is not expensive, so it makes for a poor costly signal!
If the giver knows that the receiver is treating the ring as a sign of commitment, they can decide "well I'm not really committed to this person, but I can help convince them that I am by buying them a diamond ring."
At that point, the metric has been gamed.
Because you could buy a suitable ring there (it will have an icky feeling to most people, so it will be cheaper than a "new" ring) and then sell it back when you now longer needed her to think you were committed.
Once again, I question the utility of screening based on this, even screening out on the basis of "not giving a diamond ring".
If you had a hundred billion dollars to your name, you wouldn't care an ounce about spending ten thousand on a diamond ring. Ten thousand dollars just doesn't matter. But a minimum wage worker cannot say the same. The ring is a socially established demonstration of this.
The ring is shiny, and pretty, and can be socially displayed to elicit the attention of others.
The example you provide re-affirms my point. The evaluation criteria you are reverting to is wealth. And as a heuristic for wealth, the ability to purchase an expensive ring is a reasonable metric.
As a heuristic for a good partner, it is next to useless. As a screening criteria for the complex quality of "goodness" in partner, it is likely less than useless.
If you are evaluating partners purely on their wealth and willingness to waste that wealth to enter into a partnership with you, then a ring is a more reasonable criteria.
For an assessment on more nuanced basis (e.g. does the potential partner truly love me?), it is not a very good indicator, and may even be a contra-indicator precisely because it can be gamed easily.
And that is what it is.
As a heuristic for a good partner, it is next to useless.
On its own? Certainly. But wealth is an aspect to a good partner. All else being equal, the wealthy one is better.
If you're using a diamond ring as a proxy for whether someone truly loves you, I'd suggest you're opening yourself up to be taken advantage of.
If you're using a diamond ring as a criteria for evaluating the wealth of a potential partner, I suppose that's a bit better.
I wasn't suggesting that humans were above signalling theory. Just questioning whether it had any real utility for complex evaluations like "is this person reliable and trustworthy" or "is this person going to make a good partner".