My Masked Lady ornament design called for an amethyst ... a faceted amethyst. Although, truth be known, this element - and indeed the design itself - was inspired by a visit to Phoenix, during which I purchased a set of small faceted amethyst stones. These stones had originated from the famed Four Peaks Amethyst Mine, located in the mountain range just east of the Phoenix-Scottsdale area.
So, in order to make the model as accurate as possible, I consulted the website of The United States Faceters Guild. On that site were the specifications - proportions, facet angles, etc. - for a number of different faceting styles. I chose the most basic design - a simple round form named, appropriately enough Simple Jack.
Based on that faceting diagram, I constructed a POV-Ray model that incorporates transparency, reflection, and refraction to accurately represent the play of light off of the stone's interior and exterior surfaces.
Of course, POV-Ray being what it is - a flexible, powerful ray tracing engine, I know that I am not bound by convention. While I doubt that anyone would bother to facet a piece of turquoise, POV-Ray lets me apply any texture I can dream up!
Faceted Stone Model (Ruby)
Faceted Stone Model (Amethyst)
Faceted Stone Model (Turquoise)