By the by, in case anyone doesn't know, Simon is the Aramaic word for "a grain of sand", so "a grain of sand" became, "Rock".And yet, people try to pretend there's nothing significant about Christ changing Simon's name to Peter even though they place a great deal of importance on any other case of God giving someone a new name. That's without a doubt one of the many beam in the collective eye of the anti-Catholic crowd.
I really like the fact that Christ mentions that we should build on Rock and not Sand after renaming Simon and I think people ignore that fact mostly due to the strong delusion that blinds them.
I don't know how you'd derive "grain of sand."
But etymology is tricky, puns in to languages are trickier still (the Bible is just full of puns, it;s amazing )--- and I'm no scholar.
Kēphâs --- Κηφᾶς --- a Greek transliteration of the original Aramaic word, definitely means "rock," and not any old rock, but a building block hewn and ready for construction of an edifice or monument..