The onus is not on me to prove that onyx jewelry should be seen and recognized by everyone as a symbol of Shar.
Still waiting for your proof. Jewellery made of onxy and cut into perfectly circular, flat, black discs and displayed in pride of place (such as the centre of and otherwise unadorned circlet) on the person is, in the context of the forgotten realms, a clearly symbolic choice. This is supported by the various lore that has been cited by a few people here.
It's been mentioned that onxy is generally thought of as a dark stone, or one of ill fortune by suspicious folks, and that jewellery that features it is similarly mistrusted. It's a stone that is known to be favoured by several dark gods, and their worshippers. As such, the prominent display of it in jewellery was very likely to draw attention and call suspicion onto you.
If you're disputing that lore, as cited and references already, then you're going to have to provide some evidence or proof if you want to be taken seriously.
You attempted to dispute that by implying that there was a lot of onxy jewellery lying around as standard in modules all over the place: this is not true. If you want to maintain that it is, you'll need to provide some evidence or proof if you want to be taken seriously. The evidence and lore contra to your claim has already been provided - it's your turn.
