Why stones at all?
The old idea is simple. Each graha matches one stone. Wearing that stone makes that graha stronger in your life. But a stone is only one remedy out of many. There are also mantras, giving (daana), fasting, and small acts of help. The stone 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 stone has cheaper stand-ins (uparatna). Garnet for ruby. Moonstone for pearl. Peridot for emerald. These have been accepted for a long time when the main stone is out of reach.
How a stone is really chosen
A stone is chosen from the whole chart. Usually it strengthens a graha that is already doing well, like the lagna lord. It is not used to calm a difficult one. If you strengthen the wrong graha, its problems get louder too. That is why blue sapphire comes with trial periods and a lot of care. A stone is a careful choice, never something you buy on the spot.
One warning
If someone says an expensive stone is the only thing between you and bad luck, walk away. That is a sales pitch, not a remedy. The tradition puts awareness, good conduct, and giving first. A stone can sit beside that work nicely. It was never the work itself.