What is Pitra Dosha?
The 9th house governs the father, ancestry and dharma, and the Sun signifies the father directly. When Rahu, Ketu or Saturn afflicts either, classical texts read it as Pitra Dosha - an obligation carried down the family line. It is traditionally associated with obstacles that recur without obvious cause, delays in progeny, and work that does not convert into results.
What is traditionally done
The prescribed response is not a gemstone but an act of remembrance. Shraddh and tarpan during Pitru Paksha, Narayan Nagbali or Tripindi Shraddh at Trimbakeshwar, feeding brahmins, cows and crows on Amavasya, and offering water to the Sun at sunrise. The common thread is service done in the ancestors' name rather than for oneself.