Tapering down chimney chase

[Elovia]

The solution depends on how you created your fireplace. I prefer to use soffits, but you can use the fireplace tool.

If you use soffits, you can do what you're wanting to do by breaking down the shape you want into a set of rectangles and triangles. For the rectangular parts, use standard soffits sized appropriately and apply your desired material (i.e., stone, brick, etc.). For the triangular portions, use wedges from the Geometric Shapes section of the Library, and then size and place them appropriately.

While not complete to the detail, shown below is an example that I threw together in a few minutes.

Hope this helps.