This tool reads your birth chart and names the Vedic gemstone (ratna) for each graha the chart shows as needing support. The selection is made by strength, not by birth month: it computes Shadbala for the seven planets and prescribes for every one that falls below the minimum Brihat Parashara Hora Shastra requires of it, and it adds Mars when Mangal dosha is present and Saturn when Sade Sati is running. For each stone you get the planet it serves, the metal, the finger, a weight range, the day to first wear it and the mantra.
In Vedic astrology a gemstone is prescribed to strengthen a specific planet, and never chosen by your birth month. This calculator makes that choice arithmetically. It computes Shadbala - the six-fold strength - for the seven grahas and compares each total against the minimum that Brihat Parashara Hora Shastra requires of that planet: 390 virupas for the Sun, 360 for the Moon, 300 for Mars, 420 for Mercury, 390 for Jupiter, 330 for Venus and 300 for Saturn, which are 6.5, 6.0, 5.0, 7.0, 6.5, 5.5 and 5.0 rupas. Every planet under its own line is one the chart is showing as under-supported, and a stone is named for it.
Two chart conditions add a planet regardless of its Shadbala: Mars is included when Mangal dosha is present, and Saturn when Sade Sati is currently running. Classical practice adds a further layer this tool does not decide for you - the life stone of the Lagna lord and the lucky stone of a trikona lord (the 5th or 9th house ruler) are the two most often prescribed, while the gems of functional malefics, the planets owning difficult houses for your ascendant, are traditionally left alone. That is why the same stone can be recommended for one ascendant and withheld for another, and it is worth discussing with an astrologer before you buy.
Each graha has one primary ratna, with a metal, a finger and a weekday attached to it. These are the rows the calculator draws on:
The tool also names an alternate for each primary - red garnet or red spinel in place of the Sun's ruby, for instance - because a natural, untreated primary can be far beyond most budgets. The metal, finger and weekday belong to the graha's row rather than to one particular stone, so they stay the same whichever of that planet's stones you wear.
The parts of a gemstone prescription that trace to a named text and the parts that do not are kept apart on this page rather than presented as one body of doctrine. The identity and grading of the ratnas is classical: Brihat Samhita devotes its chapter 80, the Ratna-pariksha, to identifying and testing gemstones. The mantra and its japa count are classical too, and they come from Brihat Parashara Hora Shastra's Graha-Shanti chapter, verses 19 to 20, which give the nine counts in thousands - 7,000 for the Sun, 11,000 for the Moon, 10,000 for Mars, 9,000 for Mercury, 19,000 for Jupiter, 16,000 for Venus, 23,000 for Saturn, 18,000 for Rahu and 17,000 for Ketu, totalling 1,30,000.
Two things are not classical, and the tool labels them where it shows them. The carat weight bands are twentieth-century jeweller convention: Brihat Samhita ch. 80 grades stones but prescribes no per-graha ring weight anywhere. So is the wearing ritual - washing the ring in raw milk and Ganga water, chanting 108 times, first wearing it in the planet's own hora. The BPHS remedial chapter prescribes mantra, yantra, daana (charity) and puja, not a wearing rite. Both are kept because they are what practitioners actually do, and both are shown with the note that they are modern practice rather than a chapter and verse.
For educational purposes. Astrological interpretations are traditional and not a substitute for professional advice.