Your translation of Cephas is purely invented. Cephas (Or Kepa) is Aramaic for Petra.
Petros (as a common noun) appears nowhere in the Koine (biblical) dialect of Greek prior to Christ, only in Attic Greek revival.
Word endings in Greek do not delineate substantive changes in what is being described. Petra, petrai, petrais, petran, and petras all describe the same substance, but are all feminine. Petros (or Petrws) is merely the masculinization. Rocks don’t typically have masculine or feminine qualities, but when Greek words have both masculine and feminine versions, the substance isn’t different when the endings are different, only the masculine and feminine characteristics.
(NOTE: in some post-Christian Attic-Greek revival poetry, [Imagine a modern poet using King James English], Petra was used for a mother lode, and Petros for the stone hewn from the mother lode. Given the origin from Cephas, this could not have been Jesus’ intent.)