at an angle of about 6 degrees to the standard surface portion 2 it is of course possible to spray a green paint to the slate after forming thereof thus the slate is constructed such that the reflecting angles of the light upon the standard surface 2 and the inclined surface 3 become 30 degrees and 36 degrees respectively to a viewer on the ground when an angle of incidence to the slate is 30 degrees to the horizontal in the event that the slate is fixed on a roof having an angle of inclination of about 40 degrees as shown in fig3 it is preferable to use the angle of the inclined surface portion in the range of one to 10 degrees other than the above mentioned 6 degrees but it may be possible to use the other angles the upper end 3a of the inclined surface portion 3 takes substantially the same level as the standard surface portion sb the bottom end sb of the inclined surface portion 3 is located at a middle portion between the upper and lower portions of the slate 1 the standard surface portion 2 is formed aventurine or rough and the inclined surface portion s is formed even or flat so as to increase color variations by utilizing intensity of reflection of the light due to interference of the.