Thank you for the chapter! Here's my definitely totally canon answer about that mark that is 100% true, source dude just trust.
So what she was gonna say before she was cut off was "W-well, the truth is...! I just really wanted a tattoo one day, so I went to go get one, and I thought this design looked pretty neat! But when I stripped down so they could actually apply the tattoo and they tried to give it to me, the needle wouldn't actually piece my skin, so the guy had to call someone else to help, and then SHE had to call their manager, and this entire time I'm laying there naked surrounded by a bunch of confused and slightly upset people. So, I get a little embarrassed of the fact that the needle couldn't pierce me so I run out of the shop, sprint full speed to a mithril/adamantite/orichalcum/nihonium/unobtainium mine on the other side of the continent and yank out a giant chunk of it the size of a large dog (or small horse) using nothing but my bare hands, and on the run back to the tattoo parlor I punch the chunk so much that it compresses into the size and shape of a needle. But when I make it back to the shop, I see my clothes still laying there and realize I never got dressed before I ran across the whole country and did that while thing naked and I was so incredibly embarrassed! But I was able to give them the needle, and they used it and were able to give me the tattoo I wanted, but because I forgot to sterilize the needle first I got tetanus from it, which I didn't even know you could get from that mineral. But...! That's the story of the mark I have!!!"