#macro sand(geometry, surface_texture, inside_texture, sss, absorbing, iorefraction, depth) 
  difference {
    object {
	geometry
	hollow
	translate y * depth
	texture{surface_texture}
	interior {
		media {scattering {3 sss} samples 10, 1000}
		media {absorption absorbing}
		ior iorefraction
	}
    }
    object {
	geometry
	texture{inside_texture}
    }
  }
#end

#macro sand_basic(geometry, sand_color)
	#local surface = texture {
		pigment {color rgbt <sand_color.red, sand_color.green, sand_color.blue, 0.25>}
	};
	#local backing = texture {
		pigment {sand_color}
	};
	sand(geometry, surface, backing, sand_color, 2, 1.3, 0.3)
#end