I have added a scan of some Birdstones from Cahokia Mounds to the groups files. These are unusual in form and material. As Thomas Gamut put it "scarcely any two of them are alike, and they change in form through the whole gamut". However there are three examples shown that are made from Chert. The white stone examples are made from burlington chert, and they are finely knapped with wide chert flakes removed that form the body and bird shape. Chert examples of birdstones are sometimes made from banded Slate, Porphyritic Granite, and other rare materials. Chert examples are rare.
Another example pictured is made from Sandstone. The sandstone example has twelve (12) drilled holes at all angles through and partially through, at varying angles. It is shaped carefully and has an overall beautiful form.
Recently an article on birdstones was added to the following website: