The stonebox in the midlle of the Koricancha museum in Cusco is an amazing piece of work, made out of one piece of stone. It is similar to for example the stonebox or sarcophagus in the Kings chamber of the Great Pyramid. Was it a sarcophagus? I doubt it. But how did they make it? While studying it from nearby and taking pictures, I observed similar characteristics as on the walls with the perfect fitting blocks.
The box is made on a big rectangular platform, and while focusing on the
color difference between the base and the box. On the horizontal surface of the base there is a metallic shine, it looks to me that this part was vitrified. More close up pictures do indeed show a layer that shines like metal and there is this molten look. Vitrified and molten, the same appearance as other advanced stonework at the Inca sites of Cusco!