If you wear it simply as a reminder of your grandmother, and as an object of respect for the Theotokos, without any other mumbo-jumbo attached to it, I don't see why you couldn't wear it. I have a tiny pin like this. I had no idea it was supposed to have any super "powers" attached to it. I also "inherited" it from my grandfather. He was 1000% Orthodox, but, somehow had acquired this pin. I never saw him wear it, but, I know it was in his belongings, and I took it when he died. I have it in my pretty box, with other items that are special to me. I take it out and look at it every once in a while, but, I've never worn it.
However, I have never understood how anyone can wear anything alongside their cross....on the same chain....rubbing against, covering, being equal to....the cross.
I've seen mom's who hang little baby shoes on the same chain, or medallions of little kids, others hang national symbols (in this case the Ukrainian Trident) on the same chain, etc.
To me, sharing the same chain with the cross, seems disrespectful to the Cross. How can these other items be equal to the cross?
Of course, that supposes that the cross isn't being worn merely as an article of jewelry, in which case, layer it on.
I have worn jewelry, in addition to my cross....however, never on the same chain as my cross. I use another chain that hangs below the cross.
....but, that's just me. I, too, don't speak for the Orthodox Church.