Also, that table notes Shar's symbol as "a black disk encircled with a border". If onyx is part of the lore, it's definitely coming from somewhere else. Also, the border isn't specified (here) as being purple. Also, a disc is 2d, so an onyx decoration is very different from a disk.
It's more that she is wearing a flat, perfectly circular, back bordered disc, in pride of place in the centre of her forehead - it's not part of a greater decoration, or just a coincidental fashion choice - it's a symbol, and it's really very difficult to honestly make any argument that it is not a deliberate symbol, when worn and displayed in that manner.
