I received a platinum ring, with a bright 1.2ct diamond, for re-shanking. I cut the old shank off and soldered a new one in, and, to my horror, the diamond was a rather dark brown, but the baguettes on either side were unchanged. I scrubbed the stone with a toothbrush and put it in the ultrasonic, and, after more scrubbing, the brown disappeared. There was more work to be done on the under bezel, which required more soldering (with a lower melting solder), and the stone turned brown again. Neither ultrasonic nor scrubbing had any noticeable effect, and my heart started pounding. I put the ring in the pickle bath, and thought it looked a little brighter, but I wasn't certain, but it was encouraging. More scrubbing and ultrasonic cleaning showed a definite improvement so I returned it to the pickle and then tried cleaning again. To my great relief the stone was bright again.
The ring was old, had been worn continuously, and showed much evidence of previous repairs (some with gold solder), so any deposit on the stone as a protection for the earlier repairs, had surely disappeared, or had it. I would have thought that the cleaning after the initial discolouration would have removed any deposit, so what caused the stone to turn brown?
It's OK now, by the way, I just don't know what caused the problem