being something of a homespun jewellery expert due to my expensive wifestyle
the ebst diamond substitute I've found is Moissanite (think of Moist Tonight and you get the drift) although it occurs naturally in meteorites, most moissanite jewellery is synthetic, made under heat and pressure. the result is a gemstone that looks like diamond and tests like diamond; a thermal diamond tester checks it out as diamond, scratch tests check it out as diamond, and an average jeweller wouldn't know the difference (neither would you) and you can often catch even a good jeweller out with it. I got a moissanite ring valued as diamond for insurance purposes, hey it was the jewellers fault, he's supposed to be a professional!
tip: how to identify moissanite. it's very bright and looks like really well cut diamond, also under a jewellers glass it has a double refraction, whereas a true diamond has a single refraction. it looks bloody great though, and retails up to 1/2 the equivalent price for diamonds up to about 2 carats. My mrs loves the stuff, although naturally she prefers real diamonds.
I don't see anything wrong with CZ myself