I'm also wondering about the effects of wearing a Jupiter stone during Jupiter dasa. I have a friend with Scorpio lagna and Jupiter in Pisces in the 5th. She has been wearing a very large yellow sapphire for the duration of her Jupiter dasa. She said she gained 5 lbs. total during the last 14 years of running this dasa. That doesn't seem like too much so perhaps the sapphire helped her have a positive Jupiter experience with weight gain - i.e. not too much.
I have a Sagittarius lagna with Jupiter aspecting it from the 7th house. I also have Rahu on the lagna with Saturn aspecting from 11th house. Maybe because Saturn is my strongest planet being stationary and exalted, I have been very thin most of my life. I have recently had some problems with thyroid and have gained weight for the first time. I'll be going into my Jupiter dasa next year and have wondered about weight gain then. My past experience with Jupiter transits is that I often lose weight when it transits my lagna, maybe because I feel better overall.
Is it possible that one could lose weight in a positive Jupiter dasa? Might be interesting to research.