Which is best kind of depends on personal preference. I strongly prefer moissanite, it is the most durable of those listed and has the highest fire and brilliance. But, if you want a stone that will look and act like a diamond, you want an asha. Ashadiamonds is a great company that provides a superior product. Their customer service is great.

The big factors you want to consider are the hardness (measured on the MOHs hardness scale) and the durability. Do you want a stone to span her lifetime or are you okay with replacing it every 3-5 years? A lower grade CZ will definitely need to be replaced, a higher grade CZ might last, but there usually isn’t a guarantee (if you can get one-do!) and moissanite is very hard and doesn’t scratch and cloud over like CZ does.

Here’s the MOHs for all of the above:

Moissanite: 9.25
Asha: 9.1-9.8 (internal and external)
MiaDonna: 9.1-9.6 (internal and external)
CZ: 8.25
DNL: Not available

Ok, so it looks like they all have similar MOHs ratings, all rating very high, meaning all of the above are a very high quality and durable stone. My concern then would be the clouding of the CZ. If you choose a CZ or CZ hybrid product ask their customer service assistants if you can expect clouding and if there is a guarantee in place against it. Keep this email. You will find some companies say you’re on your own, others offer a replacement guarantee and others guarantee the stone will never cloud with regular wear. It’s up to you to determine what is acceptable to you (In other words, is longevity important relative to cost).

Yesterday you were asking about man-made diamonds, I’ve dug up some reputable companies that offer man-made: Apollo, Gemesis and Chatham.

You can check out this forum for customer experiences with all of the above: http://betterthandiamond.com/discussion/ubbthreads.php

Good luck!