How to draw a 3D amethyst gemstone

How to draw gemstones and jewelrly with mixed media on gray paper. 3D drawing by Marcello Barenghi http://www.marcellobarenghi.com
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Drawing a amethyst gemstone with markers, coloured pencils and acrylic.
Final picture: http://www.marcellobarenghi.com/2013/05/amethyst-gemstone-drawing.html
Materials used: http://www.marcellobarenghi.com/p/materials-used-for-my-drawing-video-on.html
It took me 3 hours and 35 minutes
Music: "No Iron Rivers" by Dan-O at http://danosongs.com ("Package" Licence)
http://www.danosongs.com/music..../danosongs.com-no-ir