Which gemstone will best suit me?

Well said Konaji. If at all you wish to wear any gemstone, then go for Red Coral which will be very very beneficial. I will prefer to differ in case of yellow sapphire as for Sagittarius lagna, Jupiter suffers from kendradhipati dosha, hence not prescribed.

born in sagittarius lagna/jupiter.
sun sign /western is capricorn /exalted saturn.
moon sign is leo/sun.
birth star is purva phalguni 3rd pada/venus.
present dasa is of rahu from 2011 to 2029. wering ruby of sun is good as he is a benefic as 9th /bhgya lord placed in lagna.but he is in grahan yog in ketu/rahu axis.hence not advisable in rahu dasa currently running.
the best is coral of exalted mars or pushya rag of jupiter the lagna lord..
u can choose one or both.
may u be blessed.