I claim no specific expertise in the topic ... but have been researching these coins for the past few years ... and alas have not found any reliable site that details grading standards for the 50 US Classic silver commemorative type coin series.
The best I can offer you is to post clear photos of the Stone Mountain in this forum and we can offer our informed opinions.
The second best I can offer you is to judge grade based on the official ANA standards ... a quick Google search should turn up links to offer guidance.
As mentioned above, your best bet is to post photos in the classic coin grading section of the forum. You will quickly get ten or so opinions on the grade.
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