I think a bag of cement would go a very long way for making mortar for a rock wall. Why make bricks when you can just use rocks and stick them together with a little bit of cement.
Bricks were made for uniformity so you could have it as a known quantity and spit them out of an assembly line. If you're making bricks, you've already lost this game.
The video at 6:11 briefly touches on making mortar and stone construction (along with visuals):
> binding material or mortar should be of the same constituency as the brick
or stone white gypsum and sand mortar go well with sandstone mortar made of lime
clay and sand is suitable for our newly developed mud bricks
Bricks were made for uniformity so you could have it as a known quantity and spit them out of an assembly line. If you're making bricks, you've already lost this game.