Cobblestone is the fully upgraded version of a Cobblestone Frame. Cobblestone can not be upgraded any further. It can not be crafted using other materials, the only ways to obtain Cobblestone are through upgrading a Cobblestone Frame using Cobblestone Rocks or using the Creative Menu. It is not possible to dismantle, melt, scrap, or use Cobblestone as a source of Combustible fuel.
The upgrade process for Cobblestone is different to the other upgrades. There are four different stages of upgrade ranging from an empty Cobblestone Frame to a full Cobblestone block.
Cobblestone is the final result of the upgrade process and no further improvements can be made.
The full upgrade path is detailed in the chart below.
The video below shows the full upgrade path starting with a Cobblestone Frame and finishing as a Cobblestone block. It also shows how to create the materials needed to produce the frame and those used for the upgrade.