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Why did the diamond turn brown?

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

There was dirt/soap scum on the diamond that burned when you heated it. They must be very very clean before you heat. Removing can be difficult. First try soaking in liquid plumber. Put a little in a glass jar with a tight lid. put in the ring and float it in your ultrasonic for an hour or two. If that doesn’t work you can try removing the diamond and scraping the burned material off. If you got it way way to hot then the last resort before having it recut is to coat the diamond with flux and heat it up with a torch till the flux melts over it. Then pickle. If this was a fracture filled diamond then you are screwed. nothing will fix it. You will be buying them a new diamond. If it is just burned then you can have it repolished. Probably cost you a couple hundred.
Also, diamonds can burn at the temperatures that it takes to flow platinum solder. If you were soldering with even easy platinum solder near the diamond you will burn it and maybe to the point that it can’t be repolished. Diamonds should be removed in platinum rings before soldering if it is anything more than a ring sizing. Even then you need to be very aware of overheating.

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Sorry for the long delay in responding - I’ve been away.
I too thought it was a deposit but can’t understand why, after removing it the first time (leaving the stone really sparkling), it re-appeared even more tenaciously after the second soldering session. The first session was with a hard Pt solder about 3/8" away from the stone; the second, because of evidence of yellow gold solder, was with 750C solder about 1/8" from the stone. Very odd.
The stone itself remains undamaged and the repairs look fine.