I have several moissies and several diamonds of all kinds and levels of quality. In my experience, as PPs have said, to sparkle they need: 1) the right lighting; and 2) to be clean – and they all get dirty pretty darn quickly and easily. That’s why on Pricescope they emphasize cut and lighting, AND daily cleaning.
As far as comparison between stones in terms of sparkle and other aspects of visual performance, I’ve found each to have their advantages and disadvantages. In direct sunlight, spotlighting, and retail places/restaurants, every decently cut moissy I have beats every diamond I have – even my super ideal diamond – in terms of colorful fire. The H&A cuts are not as fiery, and come closer to looking like the super ideal H&A diamond. The regular round brilliant Forever Ones and my Forever One oval have more fire than the H&A moissy. My Forever Brilliant round brilliant is a sparkle bomb of a magnitude I’d never thought possible, and in those lighting settings, although the facet pattern is more splintery-looking than H&A, it has a lovely creaminess and it has depth and crispness of faceting that is more diamond-like than any other moissy I’ve ever seen. When I wear it out and about, I’ve actually had other women gasp when they saw it under spotlighting! 😉
In indirect daylight and office lighting, it’s a whole different ballgame. All of them lack the more colorful fire in that lighting, although you might catch the slightest tinge more in the moissy, but just barely. The all look their worst in office lighting – fairly dull, returning mostly white light, with a hint of haziness in the moissy (and in diamonds that aren’t cut well). In indirect or cloudy daylight or softer artificial ambient light, like a table lamp, diamonds really have their moment, with a lovely crispness, depth, and “hall of mirrors” effect that moissies, no matter how well-cut, just can’t quite capture. The facets that are “off” in any given moment are utterly black in a diamond, but are more of a dark gray in moissy – due to the double refractive nature of the material, I’m guessing – so that adds to the contrasting crispness and mirror quality of the diamond over the moissy. Also, although I have difficulty seeing this myself, many folks have noticed that light doesn’t travel through the moissy quite as quickly, so the speed of the “on/off” scintillation in the facets is slower. Additionally, the non-H&A cuts of moissy will show the shallowness of the cut as a bit of a “nail-head” in that lighting, and the older material of the Forever Brilliant will show more of the odd yellow-with-a-hint-of-green coloring.
So there you have my (rather obsessive) observations on the differences. They are definitely different animals. In terms of visual characteristics, good-quality, well-cut CZ is actually a closer mimic of diamonds to my eye. Which is the best choice for an individual probably comes down to what qualities they prize the most, and perhaps what kind of lighting they spend the most time in. I myself tend to change my rings out on the basis of what kind of lighting I’m going to be in – I pick the one whose performance I prefer to look at in that particular lighting. But then, I’m also kind of a jewelry (especially ring) fanatic, so….YMMV.