Here are mother-of-pearl and ivory (I think) side by side. MOP is hard to mistake with the silvery grey flashes under the surface. But the ivory might be bone. There are ways to tell, but like you I'm reluctant to do the hot needle test.
I am also a bit concerned now that there is the possibility that I have just imported something naughty. Although the manufacture of this pen pre-dates the CITES treaties, so I should be okay.
That hadn't occurred to me, but yes, I guess there could be some legal implications to importing ivory. The general rule seems to be that if the object is antique (>100 years old) it's legal.