Dimensions are inaccurate. Claimed as 20mm but the size is 19mm. In addition, samples of zinc, iron, and lead were quite erroneous in calculating density. I know pure substances are rare, but these alloys were too far away for my students to reliably calculate the correct density. We measured volume by displacement and l.ch. Digital scales and tripod scales for weight measurement. I looked, looked and looked again. Aggravating.