Onyx is an astrological gemstone many types of which are used as astrological substitutes for primary gemstones. For example Green Onyx is an astrological substitute for Emerald. ONYX gemstone is a type of Chalcedony which has parallel banded layers. The colors of the bands may range from white to many other shades like black, blue, green, red, red-orange, orange, honey etc. The various varieties of Onyxes are used for Astrological as well as healing purposes.