Is found all over the world in many shapes, sizes and colors.

Many agates occur as nodules in volcanic rocks (Lake Superior agates formed this way) where they started as bubbles in the lava, then filled, wholly or partially, with silica deposited in layers on the 
walls. Others form in cracks (seam agate) or eve
n in sedimentary rocks.

Agates, when cut, often exhibit a series of parallel lines, giving a banded appearance.

Such stones are known as banded agate. Other agates have plumes of color, or tree like inclusions (dendritic or moss agate).