The number seven (7) primarily symbolizes completion, or wholeness, but also covenant between God and Man, Higher and Lower. It is the day God rested, (the Sabbath being the sign of the Covenant with Adam), when he saw that all was functional - 'good.' Complete. The seven colors of the rainbow are a sign of the Covenant between God and Noah after the Flood, while the seven Sacraments are the Covenant made with the Church.
In Ptolemaic astronomy there were seven planets: Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn. Each of these were equated with the seven known metals: Gold, Silver, Mercury (Quicksilver), Copper, Iron, Tin, and Lead - the prima materia of alchemy. Hermes states in the Emerald Tablet (v.2): "What is below is like that which is above, and what is above is like that which is below for performing the miracle of one thing." The planets above and metals below are brought together in the seven endocrine glands of the body: Pineal, Pituitary, Thyroid, Thymus, Adrenal, Pancreas, Genitals. Depending on opinion these are either the material counterparts of, or the physical basis of, the seven ethereal chakras along the spine.