The same reference we grade by. No hype, no invented provenance — just what is observable in the glass, and how to check a price before you buy or sell.
Grading a marble is really two questions: what did the factory or maker leave behind, and what did the playground leave behind. Only the second one moves the grade.
Before you guess a maker, settle how the marble was built. Construction is observable. Maker is usually an educated guess.
Asking prices tell you what people hope for. Sold prices tell you what the market did. Only one of those is data.
Glass marbles survive decades of play then get ruined in a week of bad storage or one bad envelope. Here is the low-risk approach.