Coal is carbon-based, kran are silicon based. It just doesn't work, even if the energy would be readily availible to other beings. Also, on the note of coal, what things died millions of years ago before the crystal or the rock of Yliakum existed?
Iron, salt, and calcium are all reactive substances. Iron oxidizes, and bonds, salt dissolves and changes charge gradients, while calcium works similarly. Gold is non-reactive; it does not tarnish, it does not bond, it does not change charges. By superheating, you can make alloys out of it, but kran can't get to those temperatures without hurting themselves (we're talking thousands of kelvin when only a hundred kills a normal life form).