Why gems at all?
In the traditional view, each graha resonates with a gem, and wearing that gem strengthens the graha's expression in your life. A gem is one remedy among many - alongside mantra, charity (daana), fasting, and simple acts of service. The gem is the famous one; it was never meant to be the first one.
The navaratna
- RubySun ☉
- PearlMoon ☽
- Red coralMars ♂
- EmeraldMercury ☿
- Yellow sapphireJupiter ♃
- DiamondVenus ♀
- Blue sapphireSaturn ♄
- HessoniteRahu
- Cat's eyeKetu
Each has traditional substitutes (uparatna) - garnet for ruby, moonstone for pearl, peridot for emerald - long accepted where the primary gem is out of reach.
How a gem is actually prescribed
Classically, a gem is chosen from the whole chart - usually to strengthen a well-disposed graha such as the lagna lord, not to pacify a difficult one. Strengthening the wrong graha is held to amplify its troubles too, which is why blue sapphire in particular comes wrapped in trial periods and hesitation. A gem is a considered prescription, never an impulse buy.
A gentle caution
If someone insists an expensive stone is the only thing standing between you and misfortune, walk away - that's a sales pitch, not a remedy. The tradition's own hierarchy puts awareness, conduct, and generosity first. A gem can be a beautiful companion to that work; it was never the work itself.