Budha, Mercury, is the fourth graha. Its presiding deity is Vishnu, its gemstone is the emerald, its element is earth, and its Vimshottari period is seventeen years. In jyotisha Budha signifies intellect, speech, reasoning, commerce, and skill with words and numbers. It is classed as a benefic, but a mutable one, its nature is said to take on the character of whatever planet it associates with, so a Budha joined to a malefic turns malefic in effect. Budha rules two signs, Mithuna (Gemini) and Kanya (Virgo), and is uniquely both ruler and exalted planet of Kanya. Because it never strays far from the Sun, it is often combust, and the tradition weighs that closeness when judging the clarity of the mind and speech. These are jyotisha's significations; the underlying body is the innermost planet.
Graha derives from the Sanskrit root grah-, to seize or grasp, so the nine grahas are seizers of a nativity rather than planets in the strict astronomical sense, a definition broad enough to include Rahu and Ketu, calculated points with no physical body, alongside Budha, which plainly is one. Budha's inclusion in the sevenfold, and later ninefold, scheme reached Sanskrit astrology from Greek horoscopy through the Yavanajataka, a Sanskrit versification of a Greek astrological text from roughly the second to fourth centuries CE, which carried with it the dignities, exaltations and planetary week built up around bodies India had tracked for far longer on its own terms. The particular significations, gemstone and dasha length jyotisha now assigns Budha belong to a later classical layer of the tradition, standardly attributed to compilations such as the Brihat Parashara Hora Shastra, dated to roughly the seventh or eighth century CE.
Sources Brihat Parashara Hora ShastraSage Parashara (attributed) with Jyotihsastra: Astral and Mathematical LiteratureDavid Pingree, The Yavanajataka of SphujidhvajaDavid Pingree