I think I figured out how Homeworld ranks Gems. The harder a gemstone is, the higher they are in ranks. https://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness
Here are a few examples to why this theory works:
- Sapphire's are 9, therefore being right below Diamonds, and being aristocrats
- Diamonds are 10, being indeed the biggest ones in Homeworld
- Aquamarines and topazes are 8, being high-ranked in the show
- Pearls are 2, and they are literally so low-ranked that they are servants
Theres only one hole in this theory: rubies are 9, but Rubies are common soldiers.