Look at the bottom of the mural. The ground is moving around it. The diamonds around it are rocks. Also the leg is cracked. Like an earth bender(in Avatar earth benders use their feet to do their bending ), this gem, Jade or better known as desert glass, uses her feet, which is the position of her gem,to fight. She has to touch the ground to use it. That's why the desert glass has to touch the ground to use it, unlike Lapis' mirror. If you're not on the ground you can't manipulate it.