Fordite -commonly referred to as MotorCity Agate.  It was a result of paint build up on assembly line components until the build became an obstruction for the production process hence replacing parts and trashing others.  It was the discovery of factory workers who cut off and took home some of the pieces that when cut into vertically exposed the many layers and colors that had been baked on.  The rest is history.  There is less and less available today since the process of painting cars dramatically changed in the late 1970's.  

These all silver rings are designed on a saddle band (think English riding saddle smooth and wide on top and narrow underneath).

Measurements  A, B, C 1 1/4"long x 1" wide.  D 1 1//8" x 1 1/8"