They don't just look like diamond; chemically they're extremely similar, too. Diamond is just a bunch of carbon atoms covalently bonded together into a 3D crystal, which is why they're so incredibly hard. Moissanite is basically the same but it's carbon and silicon atoms mixed together. Silicon has the same number of valence electrons, so it can function similarly chemically as carbon, hence why it works. Thus, moissanite is also extremely hard and refracts light in beautiful ways, too, except imo even more beautifully. Instead of a colorless luster, it's a subtle rainbow luster to moissanite.

Source: I got my fiancée a moissanite ring, and it's lovely. And because it's lab-made, I got her blue moissanite (the coloring is just from adding certain impurities) that matches our cat's eyes perfectly. It's way more unique, cheaper, and more ethical than diamond, but doesn't sacrifice on quality one bit.

And a more interesting luster IMO