How a stone is chosen
The classical basis for prescribing a gemstone is the lagna and its lord. Each planet has a stone associated with it, and the stone of a benefic planet for your ascendant is held to strengthen what that planet governs in your chart.
Why the wrong stone is worse than none
A gemstone is understood to amplify its planet, not to improve it. Strengthening a planet that is badly placed for your ascendant amplifies exactly what you would rather it did not do. This is why blue sapphire in particular is prescribed cautiously — it is the stone of Saturn, and it is not suitable for most ascendants.
What this page gives you
This is the stone indicated by your ascendant lord — the standard starting point, and enough to know before anyone tries to sell you something else. A full prescription weighs the strength of the planet, the running dasha and the houses it rules, which is a reading rather than a lookup.
Common questions
Can I wear any gemstone I like?
Not safely, in the traditional view. A stone amplifies its planet, so a stone for a planet that is unfavourable for your ascendant strengthens exactly what you would rather it did not.
Is blue sapphire dangerous?
It is the stone of Saturn and the one prescribed most cautiously. It suits relatively few ascendants, and it is conventionally tested before being worn permanently.
Does the stone need to be expensive?
The classical requirement is that the stone is natural, untreated and free of visible flaws, and heavy enough to be effective. Price follows from those, but a high price is not itself the requirement.