I find people usually like a specific amount, and that often there is such a thing as excess, even with desirable things, traits. This also changes over time, and from culture to culture. Another example is the coloring of clothes. In ye olde times, a vibrantly bright colored article is a rarity, because dying it, and then keeping it clean are not trivial tasks. Nowadays, bright basic colors such as royal blue can easily seem cheap, like something from a bazaar. And many subtle tones, such as colors resembling what you'd find in nature, are popular.
Right. There's also the fact that cubic zirconia has, like moissanite, a refractive index higher than diamond's. So a lot of poverty-tier CZ costume jewelry is, like moissanite, extremely shiny. That moissanite is basically indistinguishable from CZ -- but highly distinguishable from diamond -- is not a point in its favor.
> In ye olde times, a vibrantly bright colored article is a rarity
I had heard that medieval nobility was very fond of brightly-colored clothes. Blues, reds, yellows, etc. It stands to reason that this was to exhibit their wealth in an immediate and obvious way.
Shininess is one property people appreciate in gems, but not the only one. Many so-called fine things are loved exactly because their quality can’t be pinned down on a simple scale, but rather is a blend of subjective evaluations whose weightings are constantly evolving as tastes change.
Is the best wine the one with the highest alcohol content? The best chocolate the one with the most cocoa or the most sugar? The best painting the one with the most contrast? Etc.