This might come across as snarky, but this is an honest question in good faith - I couldn't find my kid's 5e rule books, so I got new copies of PHB, DMG, and Monsters Manual. Neither Shar nor Selune make the index of any of them. The only mention I could find is single rows in the deities table in the PHB appendices. So, where does the lore about them that the character is expected to know come from?
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.
Based on the information available without other materials, I'm finding myself more inclined to JandK's position.
