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The original brilliant cut diamond stl had some really ugly internal geometry that wasn't even cleaned up by the ultimaker version and made boolean operations break. Not to mention it had unnecessary faces and... it was a mess. When I needed a diamond for a project I had to clean up the model, so I thought I'd share it.
EDIT: Had some edges rotated incorrectly. Fixed it.
Instructions? Use this one, not that one in any project you have. You'll thank me.