The center is and R with a difficult to read letter or number, possibly
I have a lovely gold pendant that may once have also been a brooch. It is set with amethyst. The only marking is on the back. I have included photos. It is a diamond shape with an "Ra" in the center, or possibly "Rd" or "R6." The diamond is divided into five parts. Around the "R," at the top is the number 25. To the left is an "I," to the right is an "X." At the bottom is the letter "M."