I don't know much about multi-testers. That said, I guess it could be a purple sapphire.
Do you know what type of tester was used?
Where you got it or who you got it from might hold more clues as to how expensive of a stone they'd be likely to own. So could what type of metal the setting is made of.
It could also be an amethyst or a simulant of some kind.
Maybe someone who knows more will weigh in soon. Please keep us posted.
If GIA graded that a purple diamond, mined from the earth, and the source of color was also natural, it would be worth millions of dollars, maybe tens of millions.