Agate: A fine-grained, fibrous variety of chalcedony with colored bands or irregular clouding.
Agate forms when gas bubbles trapped in solidifying lava become filled with alkali and silica-bearing waters, which coagulate into a gel. The alkali attacks the iron in the surrounding lava, and bands of the resulting iron hydroxide are created in the gel, which loses water and crystallizes, leaving the bands intact. Many agates, when cut in cross-section, reveal striking forms.
The most famous place that agate is mined is Idar-Oberstein in Germany where it has been collected since 1548. Other locations to find agate are Uruguay, Brazil, United States, Mexico, Madagascar, Italy, Egypt,
India, China & Scotland.