Before diamonds were the go-to for engagement rings, color stones like ruby, sapphire, and emerald were the thing. So why should it be mossianite and not going back to other gem stones?
Also, it only cost about 25% of my salary at that time. Spending 200% monthly salary on a ring makes absolutely no sense.
And this is my favorite https://www.stagandfinch.com/product-page/paraiba-teal-yag-2. ... And is what what my partner's stone is.
Here in France a single diamond ring is quite rare. Sapphire, emerald, ruby, etc rings are very very common. They are usually lined with tiny diamonds around to add a bit of sparkle around a large gem. However due to Americanization of the cultures, people start to do it the american way (we call it a "solitaire", solitary diamond).
Three couples in my friend group got engaged recently, all three had colored gems on their rings. Diamonds were in support of the main gem to enhance it, not the star of the show. That's a sample of N=3, but very much part of the French culture.