I cannot find a clear consensus of how a diamond is created as most of the ratio diagrams simply don't make sense to me.
What I did do is create a classic diamond that can easily be modified to specific requirements.
You will see a rather short list of features. in fact, cutting a diamond really doesn't involve too many steps. Getting it to reflect light perfectly is a different story, however. Not having a good renderer makes this one a "generic" grade and is certainly not optimised for reflections.
The point set Facet_Guides drive the half facets. You can edit these to change the profile. You will also find the diamond's girdle diameter can be set in relations. Everything else will follow this size parameter (if I got it all right).
You will also find the material colors vary for each facet using the glass setting. Again, I really can't get this to work for me but it is a starting point for randomizing the faces.
The attached file is Creo 2.0. I lay no claim to this file and offer this free to use as you may.
P.S.: If you have intimate knowledge of how to properly interpret the GIA Diamond ratios, please enlighten me.