This is a round brilliant gem model for Rhino3D. It should work in Rhino 6 and higher. If you have a Mac, try the main gem model first. If it doesn't work, try the experimental one that says it's for Macs (however, be aware that the one that says it is for Macs will not have separate layers for the gem and the cutter). An OBJ version of the gem is also available. If the other two files fail to work in Rhino for Mac, my hope is that the OBJ file will work, but if you use the OBJ file, you will need to model your own cutter. Fortunately, modeling the cutter is easy.

With the exception of the girdle, all proportions and angles are within GIA's specs for a well-cut round brilliant diamond. Diameter is 10 mm but it can be scaled to any size. I purposely made the girdle fairly thin in the hopes that it will not remove too much metal for designers who use the cutter to cut notches in prongs and seats. I have also set the pavilion angle to the lowest value of the recommended range in order to not remove too much metal under the gem while still staying within the boundaries of a well-cut round brilliant diamond. I have put the crown angle at 34% (the middle of the recommended range) because I think it looks nice and metal removal is likely less of a concern above the girdle. (I'm not a jeweler, so I'm not sure metal removal matters either way, I'm just being safe with regards to the pavilion angle.)

Proportions:

• Table is 54% of diameter. (GIA recommends 52% to 62%)
• Crown Height is 15.5% of diameter. (This is set by the crown angle)
• Pavilion Depth is 42.9% of diameter. (This is set by the pavilion angle)
• Star Length is 50% of the girdle-to-table distance. (GIA recommends 40% to 70%)
• Lower Girdle Facet Length is 75% of the girdle-to-culet distance. (GIA recommends 65% to 90%)

Angles:

• Crown Angle is 34°. (GIA recommends 31.5° to 36.5°)
• Pavilion Angle is 40.6°. (GIA recommends 40.6° to 41.8°)

Girdle Thickness:

• Girdle is 3.0% of the diameter. If I understand correctly, GIA prefers the girdle to be between 3.5% to 4.2%, but I went a little below that as the girdle (of the cutter object) is used to cut notches in prongs, again, in an attempt to err on the side of removing too little metal rather than too much.

Despite my efforts to remove as little metal as possible, each job must be taken on its own. If you are designing a piece for a gem that is less than the above standards, then the cutter for this model will remove too much metal. Usually you can scale the gem vertically to overcome this, but you may need to make a custom cutter and then use this gem only for the renders.