Complex sentence: The cast could in theory be that of any isometric mineral but in 2005 a cube of fluorite were found re-growing in the corner of a cast in perfect alignment with it, indicating that the original mineral was, indeed, fluorite.
Split into simpler sentences: The cast could in theory be that of any isometric mineral but in 2005 a cube of fluorite was found re-growing in the corner of a cast. This crystal was in perfect alignment with the cast, indicating that both came from the same source, and the original mineral forming the cast was, indeed, fluorite.