Kepha: Petros or Petra?

In the same blog I mentioned earlier, this question was posed to me: "If Christ wanted to call Peter a "rock" in Matthew 16:18-19, he would have called him Cephas, as He did in John 1:42." This was my answer: The accepted belief is that the gospel of Matthew we have in English is based on the Greek translation of what the apostle Matthew actually wrote. The witness of the early Christians, e.g., Papias, is that Matthew wrote in Aramaic, not Greek. There is no evidence from that same period to say that Matthew wrote originally in Greek. The Greek used in the oldest manuscript we have for Matthew also shows evidence of translation from Aramaic, e.g., some parts are obvious transliterations of Aramaic expressions into Greek, not naturally written in Greek. One has to grant that when Christ spoke to Simon Peter that day, he would have spoken in Aramaic, not Greek, since they are all native Aramaic speakers. Greek is a foreign language to them. The gospel of John was written in Hellenic Greek. There is no evidence to contest this. In fact, in that text of John 1:42, he makes his intentions clear: he relates that the Lord names Simon as Cephas (or Kepha/Kipha, i.e., rock), and John translates this for his Greek readers. John's usage of Petros here is interesting. He naturally knows the difference between 'kepha' (rock) and 'evna' (pebble) but he translates this for his audience as 'Petros' (pebble) instead of 'Petra' (rock). That was not a mistake. If he had to transliterate, he would have used 'Petra' but there's a problem: 'Petra' is feminine. So he gives him the masculine form, 'Petros' (just like Joseph instead of Josephine).