This graphic design is called ægishjálmur, a magical stave symbol mentioned in Old Norse literature. (It is not a Viking symbol.). There are many, many meanings, but in modern times, it represents the "Helm of Awe" or is an icon meant to help suppress the fear in one's mind.

Please note that the outside rim has a beautiful black and blue scratchy pencil pattern. I'm happy to send a photo of that rim if you want to take a closer look.