Ketu is the imaginary headless planet which represents spirituality. It
is the south node of the moon. According to the Puranas, after the
primeval ocean was churned, Lord Dhanvantri appeared with a pot of
Nectar. This was the Nectar of Immortality. Lord Vishnu was distributing
this nectar amongst the gods, when the celestial serpent demon Rahu
mingled amidst the gods so he could drink the Nectar of Immortality
(Amrita). The sun and moon gods (Surya and Chandra) saw through his
deception and notified Lord Vishnu.Lord Vishnu cut off Rahu's head in a fit of rage, but Rahu had already drunk the Nectar and was immortalized. Thus, his immortal head was severed from the immortal body/tail. Rahu and Ketu are the severed head and tail of the same demon. The immortal tail is known as Ketu or dragon tail. Ketu causes lunar eclipse. The gem of Ketu is Cat's eye.