Hallmark JM?

I have a beautiful large Sugilite(?) and Opal Pendant, downward V shaped, lots of stone, with the initials J M and the word Sterling.The back is also "stipled"like little lines of design, like little railroad track designs around the “JM” and “sterling”. Do you have any idea of who this might be? It is a stunning and huge piece, but I keep hiiting a roadblock at the initials!! Thank you very uch for your great work here, Michelle Amieux

Do you have a picture of it?