Since Sardonyx is a gemstone that has Carnelian and Onyx in it, I think that Ruby and Pearl are Carnelian while Pearl and Sapphire are Onyx.
Carnelian- Like the unexpected fire of a sunset, or the first flash of autumn brilliance, Carnelian captivates. Its bold energy brings a rush of warmth and joy that lingers, stimulating and empowering. Known as a stone of motivation and endurance, leadership and courage, Carnelians have protected and inspired throughout history.
Onyx- Onyx is said to banish grief, enhance self-control, stimulate the power of wise decision making, as well as to assist one in absorbing the energies of the universe that one most requires.
Both of these together make up the properties of Sardonyx that we see in Steven Universe.
Sardonyx -a highly protective stone that embodies a vibration of happiness, optimism and confidence. It helps to boost your self control and your level of personal power. It has excellent metaphysical properties that encourage motivation, willpower, discipline and strength of character.
Blue onyx and Carnelian above!

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Steven Universe Theories (Continued)
RandomThis goes along with my previous theories! I hit the limit on the other book.