Agate, onyx, and sardonyx are all banded varieties of chalcedony (cryptocrystalline variety of quartz) in which the principle difference is the colour of the bands. Agate is variegated generally exhibiting concentric, but irregular and ill-defined bands of white to brown, and sometimes bluish colours or other markings. Onyx has straight parallel bands of alternating white and black,and sardonyx has straight parallel bands of white and reddish-brown to red (sard). Agate, onyx, and sardonyx have hardnesses of 7 on Mohs' scale and they fracture conchoidally upon impact. Agate is generally found in pebbles in the Eastern desert and in dikes found near Wadi Abu Gerida (see map). Onyx and sardonyx are believed to be found in Egypt as well (Lucas and Harris, 1962).

   Agates have been found in predynastic graves in the form of pebbles, as well as beads of both agate and onyx. Agate was used throughout the history of ancient Egypt in jewelry as beads, pendants, and other small objects. Sardonyx was not used until the late period onward. All three stones were used extensively by the ancient Greeks and Romans in Egypt.
References

Lucas, A. & Harris, J.R. (1962) Ancient Egyptian materials and industries. E. Arnold, London, 523 p.
 
 

a) Misc objects

b) Inlay

Late Dynastic period (27th Dynasty) greywacke head of an antelope with agate and travertine inlaid eyes (height: 9 cm: Metropolitan Museum of Art, New York).