The Wiki claims, of the Diamonds' reasons for forbidding fusion:
This prevented two or more lower-class Gems from fusing and altogether becoming a different Gem, most commonly one considered higher up in the system.
This suggests that fused Gems have a rank in Gem society, which only would seem to make sense if fusion could produce non-fused Gems, or conversely if fusion-type Gems could occur naturally, since there'd be no good reason for Gems that could appear only through fusion to have any sort of social status.
Obviously, this is not a canonical source. Still, it made me wonder. We've never seen a natural occurence of a Gem that's also been seen in the fusion, right?
Can fusion between two different Gems produce a Gem that occurs normally, such as an Agate or Diamond? Or, looked at somewhat differently, can a fused Gem such as a Smoky Quartz be grown naturally, in a Kindergarten?