I'm a professional mineralogist. Onyx is a variety of Calcite, calcium carbonate. Agate is a variety of Quartz, silicon dioxide, totally different minerals.
David Garske
North side
Disclaimer: As submitted to the Arizona Daily Star.
Letter: Re: the Nov. 11. article “Mine Tales: Arizona's world-class onyx”
Onyx marble deposits just north of Mayer and adjacent to Big Bug Creek, in Yavapai County, have proven world-class.