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It would not have made it's way to the filter. A diamond is a dense piece rock and its unlikely to get sucked to the waterline, then into the filter or filter basket.My wife experienced a similar situation, and it was very upseting for her. If the diamond fell off in the tub...it will likely be there. You won't visually see it. I would carefully drain the tub down and start to feel around the intake areas and the bottom ring of the jets...things can get stuck in that area of negative pressure.
So, ladies this is warning. If your ring has not been checked in a while, it's possibe that it's allready loose and the force of jets may cause the diamond to fall out.
You reminded me of a similar situation with my wife, but we had no idea over a period of almost a day, when the stone was lost. We took a 2 million candle power spot light into the car and shined it everywhere. All of a sudden there was a sparkle coming from the carpet near the door. Nothing shines quite like a diamond.
I just make it a habit not to wear rings into the tub!