Are you building a fire pit or an out door fire place with a chimney? If its the fire place check with your local masanary store they should have a free pamphlet/paper with your local regulations with sizes, and types of materials. That should have a basic drawing with dimensions of the inside or bones of a project (don't want it crumbling around you) then you can put what ever facing you want on.
We recently went thought this process for putting a fire place in with the outdoor room we built, due to cost for us we decided against it and we are going to install a free standing wood burning stove instead.
Good luck I hope your idea works out better than ours did I would like see pictures when it's done.