Large Iridescent Stud Earrings

large 3D Flower stud earrings Gold toned and iridescent crystal detail Back of earrings has a black residue, seems like a glue because I used alcohol to clean and it is not coming out and it has a hard glue like feel. My assumption is that it is residual glue from connecting the post to the back that has discolored. size is between .5 and .75 inches Incredible sparkle in sunlight