The design of the rock changed on Jan 13th, 2017. The design was created by a person named Shaun Goodwin.
However, due to negative feedback, the design of the rock was changed back on Jan 17th, 2017. The Design of rocks is kept on the game files. http://mope.io/img/rockhill.png. Some mod packs include the old rock design in-game.
In the prior of December 28, dragons were the only one animal that could go through rocks.
Rocks are found in every biome. They are mostly found in the Arctic