3D Lapidary also has some pages on making the models:
http://www.3dlapidary.com/HTML/Intro3D.htm
about half-way down, shows how I did it before I fully understood how to interpret genuine cutting instructions. 
If you have cutting instructions to follow then look here:
http://www.3dlapidary.com/HTML/Facets.htm
The only difficulty with the latter method, is that the Boolean operations put the normals to the inside, (something I wasn't aware of at the time I wrote the page), so you will have to fix your normals at the end.

RosemaryR
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"This...this is magnificent!"
"Oh, yeah. Ooooo. Aaaaah. That's how it starts.
Then, later, there's ...running. And....screaming."