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OK, after doing a bit more web surfing I basically found somebody that's done this problem already, calculating for ice vs stones. http://scottf.wordpress.com/2011/12/20/whiskey-stones-coolin.../ Again, note there is no place in the formula to consider shape. He does not calculate the rate of change, either, but it should be clear that spherical shape is the slowest possible (least surface area for heat exchange.)