Ruling Planet.
Also known as: Planetary Ruler
The planet traditionally assigned to govern a zodiac sign, lending its qualities to the sign and to the stones associated with it.
In Western astrology each zodiac sign has a ruling planet: the celestial body whose qualities the sign is said to express most strongly. The classical assignments, fixed in the Hellenistic period, used the seven visible "planets" (the sun, moon, Mercury, Venus, Mars, Jupiter, and Saturn) and distributed them across the twelve signs. Modern astrology adds Uranus, Neptune, and Pluto, giving Aquarius, Pisces, and Scorpio newer rulers alongside their traditional ones.
The classical pairings still drive most stone-and-sign correspondences. Mars rules Aries (and traditionally Scorpio); Venus rules Taurus and Libra; Mercury rules Gemini and Virgo; the moon rules Cancer; the sun rules Leo; Jupiter rules Sagittarius (and traditionally Pisces); Saturn rules Capricorn (and traditionally Aquarius).
Each planet carries its own family of associated stones. Lunar stones tend to be milky and shimmering (moonstone, selenite, pearl). Martial stones are red or fiery (garnet, ruby, carnelian). Mercurial stones are quick and clear (citrine, blue lace agate). For anyone choosing a stone with their chart in mind, working through the ruling planet often gives a more nuanced fit than working from sun sign alone.