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Typical chunks of Cobden banded chert. The chert forms in the shape of cannon balls but you may only find a section of a ball. It also forms in double nodules or slightly misshaped ones. It's cortex is usually yellowish. Typically, it has concentric bands in the chert, itself. Heat altering usually does not work, except on very thin flakes, and is not necessary. Cobden's color varies from dark blacks and blues to lighter grays and blues. It also occurs in some tan shades. Rarely, it may have some reddish inclusions.
A ball-shaped piece of Cobden and an irregular-shaped piece, showing the appearance Cobden has as you would see it in the creek. This shows it's water-worn and weathered cortex. Cobden comes from the St. Louis formation and has good workability.
A section of a large ball showing the weathered chert. Cobden ball sizes vary from 1/2 inches to well over a foot in diameter.
For more information, check out the Parkland College CHERT TYPES pages.