Mint marbles are graded 9.0-9.9 (Mint Minus is 9.0-9.2 and
Mint Plus is 9.7-9.9);
Near Mint marbles are graded 8.0-8.9 (Near Mint Minus is 8.0-8.2 and Near Mint Plus is 8.7-8.9); and Good marbles are graded 7.0-7.9 (Good Minus is 7.0-7.2 and Good Plus is 7.7-7.9). Finally, Poor (called "Collectible" by others) marbles would be graded 0.1 to 6.9, though few people would attempt to place an exact numeric value on a marble falling in this wide range. I assume a "0" marble would be mere dust!
citation:
Well known publisher and collector
I never throw stones at "enterprising capalists" whahahahah!!