I thought about casting them, but casting translucent items is costly, messy, difficult and hazardous. I thought about cutting out individual pieces of plastic and gluing them together, but that would be time-consuming and would be difficult to make the seams look good.
I had a flash of inspiration when I thought about vacuum forming. I could easily cut a piece of wood to look like half of the gem, vacuum form two halves, then glue them together. This would also make it dead simple to repeat for all three rupees, making each of them match.
Step 1: Making the Form
To make the rupee form, I used a piece of oak I had in stock. I wanted something hard that I could get clean lines on. I cut the shape out using my table saw, then set the blade at an angle to cut the bevels into the edges. It only required very minor sanding to clean it up.