i really don't get the topic here

Hiya kicker. Well, I guess this is one of those cultural things where you have no frame of reference that would let you "get it".
In America, there used to be (and still are, but not as many) rock shops scattered around like boulders in a field. Why I still remember that shoebox of rocks that I had as a kid. And of course, we invented Rock & Roll music.
But the really big rock related event in American history, that ultimately changed our whole society,... That was the national frenzy that took hold of the Nation in the early 70's in the months leading up to a Christmas.
Marketing was still really in it's infancy. Sure they were learning new advertising stuff like "Jingles" sell and refining "Sex" sells. But they hadn't caught on yet that if you take a totally absurd and improbable idea and market it right, you can make the masses buy and do anything.
The breakthrough event was, The Pet Rock. That's right, they sold rocks to people. ROFL. And the people of America en mass bought them, and then went on to exchange them as gifts to each other at Christmas. Pretty much everyone in the country bought one.
Well, the rock mania has certainly died down since them. But the idea of having a rock still is embedded in part of the American dream. Witchy tapped into this subconscious cultural desire "for a rock", by listing a "Rock" as one of the sold out items in the new Texaswheel Swag Shop.
Joe saw this, and for some reason went on a campaign for his own rock. I assume he lost his rocks for some reason a long time ago. Or maybe he watched the cartoon with Fred Flintstone and Barney Rubble way to much as a child.
In any case, we have just been having fun with the whole concept of "rock" ownership. And I think it is because of fond memories of our own first, since lost "pet rock", that we still don't believe we actually bought at one time.
I hope that this has been helpful to you my Philippine friend.
P.S. Plus it was fun to yank on Joes chain a bit.