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The reason you can't open the 3dm file is that it was likely saved in a newer version of Rhino (like Rhino 6 or 7). Rhino 4 cannot open files from newer versions. You can either:
1. Ask the seller to 'Export' the file as 'Rhino 4 3dm' for you.
2. Update your software to a newer version.
Regarding the stones not fitting, since you have the STL, you might need to adjust the offset or scale the setting slightly in your software before printing to compensate for resin shrinkage.
