Whether it’s an error of placement by Homeworld or gemetic defect, sometimes Gems don’t form perfectly. They claim they’re “Just a Rutile that came out wrong.” This can happen accidentally, naturally, and result in a Gem who doesn’t fit. We currently have several pieces of evidence that Homeworld’s Gem production is frequently imperfect, right? But I’ve got an idea of how everything canon up to this point could still point to Sapphires and Rubies coming from a Corundum group by in-show logic as well. Now, this is not an outright THEORY theory, because I wouldn’t say I actually think this is likely to be the canon explanation. My question is, how could the show make Sapphires and Rubies so different in Gem society but use real-world gemstones that are so similar to each other? Besides making a very cute case for how Garnet’s Gems are so predisposed to being compatible, it doesn’t make much sense story-wise if gemology is taken into account. In Guide to the Crystal Gems, Jasper, Steven/Rose, and Amethyst are identified as “Gem Type: Quartz” but Ruby and Sapphire are not examined in factsheet format, and Garnet’s Gem type is simply listed as “Fusion.” We know Homeworld treats Sapphires and Rubies very differently, but we don’t know yet what they call their group (if it’s anything different from simply “Sapphires” and “Rubies”–it’s happened before, as in the case of how Peridots are their own class of Gem in Guide to the Crystal Gems as well). The show hasn’t identified (as of this writing) whether Sapphires or Rubies belong to a larger class the way Jaspers, Carnelians, Rose Quartzes, Amethysts, and Agates belong to the Quartz class.

Sapphire is “a rare aristocratic Homeworld Gem.” And Ruby is a “common soldier.”

Ruby and sapphire corundum