Heading 2515 covers calcareous stones (marble, travertine, limestone-based) with a specific gravity of 2.5 or more, while heading 2516 covers non-calcareous igneous and sedimentary stones such as granite, basalt, and sandstone. The mineralogical composition — silicate versus carbonate — is the primary distinguishing factor.

Porphyry is classified under subheading 251690 alongside basalt and other monumental or building stones. Commercially, it is valued for its hardness and decorative appearance, used in paving, cladding, and architectural applications. Its classification here applies only in crude or sawn-block form; polished or fabricated porphyry articles move to worked-stone headings.