Diamond, it turns out, is a geek's best friend. Not only is it the hardest substance known, it also has the highest thermal conductivity - tremendous heat can pass through it without causing damage. Today's speedy microprocessors run hot - at upwards of 200 degrees Fahrenheit. In fact, they can't go much faster without failing. Diamond microchips, on the other hand, could handle much higher temperatures, allowing them to run at speeds that would liquefy ordinary silicon. But manufacturers have been loath even to consider using the precious material, because it has never been possible to produce large diamond wafers affordably. With the arrival of Gemesis, the Florida-based company, and Apollo Diamond, in Boston, that is changing. Both startups plan to use the diamond jewelry business to finance their attempt to reshape the semiconducting world.