In order to examine a gem stone 1, a parameter of the stone is determined by rotating the stone, providing an image of the stone, as seen radially, sensing the edge of the image at different angular positions of the stone, and using information derived from the edge to determine the parameter. The parameter can for instance be the girdle diameter, table diameter or crown height, etc., and the invention enables one to examine without difficulty small stones, specially small stones which would prove very difficultto examine manually, whether cut or uncut.
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