Cat's eye
Cat's-eye is the gemstone recommended for Ketu; specifically Chrysoberyl cat's-eye. The finest quality is called "milk and honey", as the stone is a honey color, with the eye white, like milk. The darker varieties are generally a greenish body color and still very powerful in their effect.
Dark, especially black imperfections should be strictly avoided in this stone (as with most others) as wearing such a stone can cause early demise, as several ancient texts state. The source countries for cat's-eyes are India, Burma, Sri Lanka and Brazil.
Cat's-eye is said to increase powers of perception and insight. It should also give greater discrimination in action. The ancients believed that cat's-eye protected individuals from epidemics or diseases, from drowning and from enemies, as well as heightening psychic perceptions. Coincidentally it is supposed to bring good fortune to gamblers. Apatite cat's-eye may be a less costly substitute for cat's-eye Chrysoberyl, although its effects are less powerful, as well.