For something such as Sankara Stones I suggest using Chavant Clay, Not only can you Sculpt it to look like a real Stone but you can Polish the Clay when you have completed the Main forms.
Chavant Clay is what they use for Car Prototypes as well as Robocop's Armor and you can Polish it to a Smooth and Shiny Finish just Like the Sankara Stones in the Movie, Then you can Mold them easily and cheaply by doing a simple Brush-up Mold with a Plaster backing.
Chavant Clay is what they use for Car Prototypes as well as Robocop's Armor and you can Polish it to a Smooth and Shiny Finish just Like the Sankara Stones in the Movie, Then you can Mold them easily and cheaply by doing a simple Brush-up Mold with a Plaster backing.