But then I thought: why not simplify it more?
You have three categories, as defined by the Jovial Priest:
- Low-end gems (<100gp): are usually opaque, with exceptions (quartz;)
- High-end gems (>500gp) are usually semi-transparent.
- on a 1, you have really good quality gems from the lower category, if there is one (for the mid-range, you can change this to 1-2);
- on a 6, you have poor quality gems from the higher category, if there is one.