grade actually just refers to the tolerance for "sphericity" or roundness in all dimensions, grade 25 is spherical to within 0.000025 of an inch. Also the grading specifies the roughness of surface finish, so grade 25 have a "Ra" number of 2.0 or better, where lower grade (which have a higher grade number such as grade 100) will be rougher as well as less perfectly spherical e.g. grade 100 have a Ra of 5.0 or better.
I've only ever seen and bought grade 25 balls that were made of chrome-alloy steel but the material of the ball is not necessarily grade-specific.