High quality diamonds look grayer/silvery under bright light, whereas CZ produces rainbows. I own both, and it's easy to tell the difference if you're, say, outdoors in the summer, or under full-spectrum lighting. Moissanite is closer, but moissanite still has more of a "rainbow" look, and is often yellowish.

If a friend of mine was showing off her engagement ring and it was refracting a little rainbow on her finger in the sun (like this http://upload.wikimedia.org/wikipedia/commons/b/bd/CZ_brilli...) I obviously wouldn't say anything, but come on. Getting a CZ engagement ring is going to lead to an awkward situation at some point.

>> A CZ is indistinguishable from a diamond until you look at it with a microscope

Not true, cz has a visual 'fire'. Plus, most women can tell at a glance.

There are a few quick tests that can quickly separate a CZ from a diamond. Exhale on a similar diamond and it clears up much faster because of diamonds high thermal conductivity.

More importantly, CZ is far less though than diamonds so they don't keep their shape under abuse nearly as well. There cheap enough you could swap out the stone every year, but just keeping the same CZ in a ring does not work over a lifetime.

Because a CZ can be easily damaged, whereas the hardness of a diamond makes it very resilient to scratches, damage etc.

Also, the beauty of a ring doesn't solely exist in the diamond, the setting is a thing of beauty - the piece of jewelery itself.