The simplest way to create an STL file of a diamond is to use a 3D modeling device like ScanoxCAD. There is a wide range of software available, some of which are free, such as SketchUp, Blender, and Autodesk Fusion 360. Most 3D modeling programs have a 3D shape library that includes a diamond, so you can simply drag-and-drop the shape into the workspace but if you need to get STL from real stone the you need scanner like ScanoxCAD. Once you have the diamond shaped the way you want it, you can export the file as an STL, to Rhino or Matrix design software
