The cat's eye is a gemstone for Ketu. Some points relevant to this stone are as under.
[18] AN ALTERNATIVE OF CAT'S EYE FOR WEARING
Due to any reason, if someone does not want to wear a cat's-eye, or cannot find a suitable cat's-eye, then he or she can go for tiger's-eye. Although they are from different varieties, they are similar in other ways, so a tiger's eye can act as a substitute.
Some astrologers recommend a moonstone as an alternative to cat's-eye. However, we strongly feel that this is wrong, as Ketu is an enemy of the Moon, and the moonstone is considered to be a stone representing the Moon.
[19] WEIGHT OF CAT'S EYE FOR WEARING
The weight of a cat's-eye for wearing purposes should be
not less than 0.5 carat. This is a minimum, and a cat's-eye weighing less than 0.5 carat will give negligible effect to the wearer. A very good weight for a cat's eye is 4.0 carats. Although a stone weighing more than this will give a stronger effect, we can say that 4.0 carats is generally quite satisfactory for wearing purpose.
For the remaining 23 points, you can contact us or you can refer to our book “Astrological Science... Your Birthstones and You".