added crystal_ball

docs/lib-crystal_ball.md

@@ -0,0 +1,49 @@
+# crystal_ball
+
+Uses spherical coordinate system to create a crystal ball.  
+
+![Spherical coordinates (r, θ, φ) often used in mathematics](https://upload.wikimedia.org/wikipedia/commons/d/dc/3D_Spherical_2.svg)
+
+Dependencies: `rotate_p`, `cross_sections`, `polysections`, `ring_extrude`, `shape_pie`.
+
+## Parameters
+
+- `radius` : The radial distance r. 
+- `theta` : The azimuthal angle. It defaults to 360. It also accepts a 2 element vector. The first element of the vector is the beginning angle in degrees, and the second element is the ending angle.
+- `phi` : The polar angle. It defaults to 180. It also accepts a 2 element vector. The first element of the vector is the beginning angle in degrees, and the second element is the ending angle.
+- `$fa`, `$fs`, `$fn` : Check [the circle module](https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Using_the_2D_Subsystem#circle) or [the sphere module](https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Primitive_Solids#sphere) for more details. The final fragments will be a multiple of 4 to fit edges.
+
+## Examples
+
+	include <rotate_p.scad>;
+	include <cross_sections.scad>;
+	include <polysections.scad>;
+	include <ring_extrude.scad>;
+	include <shape_pie.scad>;
+	include <crystal_ball.scad>;
+	
+	crystal_ball(radius = 6);
+	
+	translate([12, 0, 0]) 
+	    crystal_ball(
+	        radius = 6, 
+	        theta = 270,
+	        $fn = 12
+	    );
+	
+	translate([24, 0, 0]) 
+	    crystal_ball(
+	        radius = 6, 
+	        theta = 270,
+	        phi = 90,
+	        $fn = 12
+	    );    
+	    
+	translate([36, 0, 0]) 
+	    crystal_ball(
+	        radius = 6, 
+	        theta = [-30, 270],
+	        phi = [30, 60]
+	    );        
+
+![crystal_ball](images/lib-crystal_ball-1.JPG)