Dang! Can you catch a fingernail on it? Wondering if its a glass flaw or a crack...
It wouldn't happen to line up with the washers on sides of form? I had them too tight once and the glass couldn't lay down. Didn't crack it but made a fat spot in the limb at 2 of them.
If its a crack you can catch a nail on, I would try to get some glue in it, won't be easy. What part of limb? Bending or not ?