Diamonds are an animator’s best friend or something like that. You can quickly make a nice round-cut diamond with the Gemstone tool which is located in Create > Primitive: More.

On the numeric panel you can control various aspects of your diamond. Center sets the center position and Radius the overall radius of the object.

Higher Symmetry values increase the number of polygons used on the crown and lower values decrease the number. The Crown, Girdle, Table and Pavillion settings control the size of those areas on the diamond. (See illustration below for reference.)

Make UVs Option

The Make UVs option at the bottom of the various primitive numeric panels assigns some default UVs based on the geometry of the object. Note that a UV Texture map must be currently selected or this option will be ghosted.

Make Air Polys will create duplicate polygons facing inside the Gemstone. These polygons should be used when surfacing glass or any kind of refractive material to give a more realistic refraction to your object.