There are many different rock formations to be able to be made as they are organic and naturally made over time due to geographical abrasion. But what they do continually consist of is a base shape. For our model, we will be starting with the cube primitive as our base shape.
With this shape, we have the base for our rocks which we will be creating into cave walls, debris and rubble.
A good place to start with creating Low-Poly objects is implementing subdivisions. This way we have more control over how the shape will form. To create subdivisions in your mesh, hold down the right mouse button with your mesh selected, now you can select your shape name button and go to attribute editor. Set the subdivision width, height and depth to 5.
Now we have the subdivisions in we can begin to manipulate the cube into a more intricate, natural looking object. Since rocks are basic shapes we can play around with the vertices, faces and edges to get our desired shape.
A good tool to help with this is soft selection, soft selection creates more organic deformations of the cube primitive. It works by having all the polygons in the general area deform less than the poly you selected. If you didn't use soft selection, you can get sharp edges and unnatural stretching.
To use soft selection, click on your object and in the Tool Settings tab, find the Soft Selection drop down menu. Check the box, press F8 for vertex selection and now you can start using soft selection.
To use the Soft Selection tool, you can either use the Falloff radius slider on the left or you can hold B, click and drag your mouse left and right to increase and decrease the slider as a shortcut.
Another useful tool for soft selection is the Falloff curve, It is found just underneath Falloff radius. With this tool, you can select how Soft Selection determines where the surrounding polygons defined with Falloff radius are manipulated.
After the model is finished we can triangulate the mesh, this will use triangles instead of quads for creating the mesh which in turn, makes it look a lot more like a Low-Poly model!