Agate 101 is a slide show primer on basic agate information and terminology. The information presented includes the defining characteristics of agate, different varieties of agate, and the basic structures of banded, moss, and plume agate. For a more thorough study of agate, see the “Speaking of” page on the main menu.
To get you started off, I will dispense with the chemistry part: agate is chiefly composed of silicon dioxide, also known as silica. Silica occurs in many forms, and is the most common compound in the earth’s continental crust. One of the common forms silica can take is the mineral quartz. Agate can be thought of simply as a form of the mineral quartz, but with an almost endless range of fascinating variations.