IMPORTANT:
If you refine an Awakened Radiance Stone with another Superior or Heroic Radiance Stone, there is only a chance to obtain either an Awakened Radiance Stone OR a Legendary Radiance Stone. Even though the UI says it is a 100% chance to obtain a Legendary, because Awakened Stones are also Legendary, it is actually a chance to obtain either.
If you refine an Awakened Radiance Stone with another Legendary Radiance Stone, then it is a 100% chance of obtaining an Awakened Radiance Stone back.
both option include already having an awakened unity stone and not making one