Cubic Zirconia looks just like a flawless diamond. Few people can tell the difference with a naked eye inspection. A cubic zirconia engagement ring will look just like an diamond engagement ring. I use cubic zirconia in almost all my samples and mock-ups.
If you live in a rough neighborhood or will be travelling on your honeymoon to a high-theft area, I strongly recommend having a cubic zirconia replica made of your valuable engagement ring. It will look the same to all and can be lost or stolen without worry.