Diamonds are useful for their hardness, and they wouldn't block as much light as a colored gem. But again, it's about the marketing.

...Well, Noriko seems perpetually teenaged in mentality, and she's worshiped as a goddess by some... so it's not that far fetched.

Of course, being a Xenoi I don't really believe in a particular 'god'... the word doesn't sit well on my tongue. But as far as humans understand the word, it could be appropriate.