Flint and chert are both in a group of silica containing rock formations that are often bedded within layers
of sedimentary rock.  Scientists theorize that many of our good knapping cherts and flints are the results of
ancient marine life and/or coral features that used to live in long extinct oceans which covered much of what
is now dry land.  While searching for artifacts one day, I came accross a drumlin land feature near
Rochester, New York, that was covered in
Onondaga Chert.  None of it was of knapping quality, but I did
find several pieces that were in the shape of coral.  The outside cortex was a brown sandstone type of
material, while the center core was solid chert.  The higher the silica composition, the nicer a flint or chert is
to knap.  Flints are generally near pure silica in content while cherts are more impure and can be of a slightly
lower silica content.  The closest to pure flint that can be found in the United States would be Texas
Georgetown flint.  It's white chalky cortex and flint color are very similiar to some of the fine European flints.