Here's pictures of two big fake diamonds I made in 2011. I followed an old Belgian diamond cutters manual, Diamond Design, by Marcel Tolkowsky, published in 1912. I was using his recommendations on geometry and proportions for "correct" cuts of modern brilliant round diamonds. In my attempts, this is what happened, they are not correct in form, but as a generic form, they still read as diamonds. One diamond is made from 1/2" mdf, and the other is made from 3/4" knotty pine plywood. Both are fabricated, hollow forms.
Here is the diamond on a table I also made, the diamond wasn't finished yet, it was bare mdf.