Agate is a microcrystalline variety of quartz (silica), chiefly chalcedony, characterized by its fineness of grain and brightness of color. Although agates may be found in various kinds of rock, they are classically associated with volcanic rocks but can be common in certain metamorphic rocks.
Surfing the net, there are many variations in color and texture. A wonderful tutorial is found at http://www.polymerclaycentral.com/faux_agate.html . I really enjoyed grating the cold clay and shaping the balls of clay. The translucency in the final baked product has to been seen to be enjoyed. The top left-hand image is of real Agate.