This theory is extending upon the Pearl and Chalcedony theory.

Chalcedony is a type of Quartz. I looked into all of the fusions with Pearl and they all have a direct connection with Quartz.

Sardonyx- Type of Chalcedony that has Quartz.

Alexandrite- Always found with Quartz.

Opal- I believe she is actually Opal Aura Quartz also called Angel Aura Quartz. She looks like an angel and she was formed at the sky spire hunting the heaven beetle. The color's also match perfectly.

Rainbow Quartz- Is Quartz.

My theory is that Pearl added the Quartz part not Rose. Pearl is actually Chalcedony and her fusing with others make them a fusion with a Quartz. I believe that with Rainbow Quartz they were trying to make us believe that Rose Quartz fused with Pearl and that's why Rainbow Quartz has Quartz on the end. Because of Rose. I think it was to throw us off of the Pink Diamond theory. It was Pearl not Rose that made Rainbow Quartz what she is. Pearl made Rainbow Quartz into Quartz. Pearl is made of Quartz and it's shown in every fusion we've seen.