Agate is a banded, fine-grained silica - the same composition as quartz, but the grains are so fine, you can't see them with the eye or with a hand-lens. By definition, agate is banded. It is a common form of silica, but some places are known for the beautiful banding
Pabian, R., Jackson, B, Randy, P and J Cromartie, 2006,Agates, Treasures of the Earth,Firefly Books, 184 pp.
Polka Dot Agate Mine
Oregon, United States
These pure white beads are from the Pridey Agate Beds of central Oregon, well known for the brownish ‘polka dots’. This Snow Drift agate does not show banding but is cut from the same rock.