The idea that onyx is not presented as jewelry all throughout the history of DnD (and specifically within Forgotten Realms) is patently silly.
Associating it with evil and assuming everyone in the setting associates it with evil (despite the mention above about how it reduces the complications and pains of childbirth) is ... well, I'm happy for people to think what they will about that.
We could talk about treasure tables in Myth Drannor if we want and find onyx jewelry there. We could point to the Onyx of the Richest Wort, which was an enchanted necklace that boosted luck. But really, at this point, it feels ridiculous. Arguing that onyx isn't used commonly as jewelry is like arguing that feet don't walk. I might as well be banging my head against the wall.
So, again. Thank you for your interpretation. You think I'm wrong. I think you're wrong. I think that about says it all.
