Yes that's true, but I'd presume a lot of what you pay for in a diamond ring initially is not the cost of raw materials, but the symolism, and hence a diamond ring would lose value.
I was thinking you'd lose money either way if you went with diamond vs. non-diamond, so "you'll lose value" is not a compelling argument for non-diamond.
Everything about a wedding is front-loaded to an exreme. From photos, to flowers, to cars - when it's a wedding the price is loaded. It's a complete rort, everyone knows it is going on, yet - there it is, everyone just pays up. Because nobody wants to be seen to be cheap.
I was thinking you'd lose money either way if you went with diamond vs. non-diamond, so "you'll lose value" is not a compelling argument for non-diamond.