Updated formulas are presented for estimating the weights of mounted colored gemstones. These formulas are derived from measurements and weights of thousands of German-cut calibrated amethysts and citrines, representing most commercially available shapes and sizes. As with the formulas taught by GIA, the dimensions of a stone are multiplied by its specific gravity and by a “shape factor” that is determined by the stone’s face-up outline. This article also illustrates how the shape factor changes over a continuum of common face-up outlines. As in previous formulas, a separate weight correction factor is applied to stones that show proportion variations in profile view.
