Needed to make a gem for a game so I decided to make a very quick 'how to draw' tutorial.
1. Draw a box
2. Select box, press "Q" (for transformation tool). click a corner, hold "SHIFT" and rotate to a perfect 90 degrees
3. Cut the top corner off
4. Fill the middle with a bright color
5. Use the line tool to divide the center shape into several shapes, fill them with different shades of the same color and delete the lines.
6. Add some details to make it look shiny