Abstract: The invention relates to a corner joint for building units of stone, especially of soapstone, for furnaces or the like to join two units (1, 2) cornerwise to each other by using a binding hook (3) extending from one stone unit (1) to another (2), which binding hook is arranged to sink into a groove (4) cut in an edge (14) of the stone units for this purpose, while joint pins (5) of the hook sink into borings (7) provided for them, the joint comprising a corner piece (8) separate from the units (1, 2) and having two joint surfaces (9, 10) forming an angle with respect ot each other, against which surfaces joint surfaces (11) of the units (1, 2) are arranged. To carry out the installation of the corner joint reliably and effectively, at least one end face (12) of the corner piece (8) is provided with a groove (13), into which a web portion (6) combining the joint pins (5) of the binding hook (3) is arranged to sink in order to lock the units (1, 2) and the corner piece (8) together.