"As long as you are standing on stone or unworked earth (including normal soil), you can use the Heighten Spell feat to added effect."

In terms of game mechanics, "standing" is the opposite of "prone" so as long as you're not prone, you qualify for the standing portion. Whether or not any dirt in your shoes is considered "worked" is irrelevant if you get shoes/sandals with the soles made from stone slabs, as stone doesn't matter if it is natural, carved, or otherwise worked. As long as you aren't prone you're considered standing, and if your feet are on those stone slabs you're considered to be standing on stone.


Since most builds use Earth Spell with Signature Spell to spontaneously convert Silent Image into any Shadow spell, you may as well take Ability Focus: Silent Image from the good ol' Monster Manual to get +2 to all your offensive spell DCs.