Ultrasonic Diamond

How can I remove hard water stains from gold/diamonds?

We have very hard water with no water softener. Over the past year, my engagement ring has built up what seems to be stains from the iron in the water. My diamond looks yellow, and it isn't supposed to! I've tried jewelry cleaner & had it cleaned at the jewelry store with the ultrasonic, but it's not working. What can I use that will not harm the gold or diamonds?

use perhaps the Riemann's dzeta function