Hi - "Plat" is one of the marks used for PLATINUM. 523 sounds like a millesimal value, usually you see on gold. Maybe the ring is combination of Platinum and Gold which is fairly common. Examine the metal, and see if you can notice any change or different tones of color. Platinum is often used around a diamond setting because it's so strong and Plat also enhances the brilliance of a diamond. cheers.
