N.Y.C. Admin. Code § 27-862: § 27-862 Materials other than metal for construction of chimneys.

Materials other than metal for the construction of chimneys shall include the following:

(a) Clay, concrete, or shale bricks, laid in a full bed of mortar.

(b) One hundred per cent solid masonry units, or hollow masonry units having the cores filled with mortar or grout, laid in spread mortar covering the entire cross-sectional area of the unit.

(c) Concrete cast in place and adequately reinforced where required.

(d) Natural stones that have been sawed, dressed, or have a tooled finish, laid in spread mortar.

(e) Perforated radial brick. Such brick shall be laid in spread mortar equal to type M, of reference standard RS 15-3, and shall be shaped to the circular and radial lines of the various sections so as to form even joints.

(f) Other equivalent materials or combination of materials.