It means that its only gold plated
The jewelers mark means it has just been marked by a certain person. Think of it this way a brand you brand cows with is a mark of a farmer so he knows what cows are his. That is what a jeweler does.
Jewelers' Circular Keystone
MJ likely stands for the initials of the manufacturer or designer of the ring. It is common for jewelers to include their initials or logo on jewelry pieces as a form of identification.
Atsuko Seta was born in 1955.
SETA Corporation ended in 2009.
Seta Tuilevuka was born on 1981-10-27.
Seta Hagopian was born on 1950-07-28.
Marek Seta was born on 1987-07-26.
Détour De Seta was created in 2004.
SETA Corporation was created on 1985-10-01.
Most likely it is Gold shell (gold plated) But it could be a jewelers mark.