I'm talking about something like this:



How could I make this kind of crack in Houdini? I've tried RDB Material Fracture, which cuts the mesh into separate pieces. But I don't know how to glue them back together and make these "grooves".

I think you'll want to create curves for these kinds of cracks. You can then give them thickness and bool into the geo.
You could try with the Find Shortest Path node. Python or VEX would also work, anything that lets you generate branching curves.