Musselshell River Rock

[salvageon]

 

[DDancer]

There are a number of things it could be but my guess is that its a sandstone, its not uncommon for the material to break up a bit ~ fracture~ with strait cleavages and for the resulting cracks to fill with another mineral. Most common is a calcite mineral. As most sandstones are cemented by calcite minerals they tend to fracture like calcite does with strait cubit lines. Calcite then dissolves from the base material and fills the cracks.
Other types of rocks do similar, basalts, granites and some ingenious rocks and each can have a different filler between the gaps.