My theory that not all gems are made for a purpose, and that not all fusion gems are exclusively fusions could be backed up by new evidence from the canon.
We’ve learned that quartz gems are warriors. “Broad shoulders, intimidating” Well, you know who’s a quartz but is almost the opposite of how Peridot describes a quartz warrior, and is also a fusion involving no quartz gems?
Sardonyx is a quartz, but Pearl, Ruby, and Sapphire are all not quartz.
So, fusion gems are different to normal gems.
A normal Sardonyx (which were most likely made in the kindergarten) would be how Peridot describes a quartz. But fusion Sardonyx is not. When gems fuse, they can ignore the “rules” of what a gem should be. THis is probably why homeworld looks down on them so much. They go waaaay out of the norm. I mean, no offense to Sardonyx, but she’s not exactly intimidating. Sardonyx is 3 gems, none are quartz, but Sardonyx is; and she defies everything we’ve been told about quartz gems.
So basically. There can be gems like Sardonyx, Sugilite, or Garnet that are just one single gem, and not a fusion. However, the fusion versions will be vastly different from the single gem, as they’re made of different gems, who all have their own traits. That’s why homeworld hates fusion, and sees it as a weapon. Because fusions go so against the strict norms that their society is built on.