Stone is not combustible and will not support or encourage the spread of flame. Being non-combustible stone is therefore considered a fire resistant material.Because of its thermal conductivity, heat transfer is fairly rapid. Most stone is not considered a highly rated thermal insulator.
A comparative measure expressed as a dimensionless number and derived from visual measurements of the spread of flame versus time for a material tested in accordance with ASTM E 84 (Standard Test Method for Surface Burning Characteristics of Building Materials). Under ASTM E84, interior finish materials are rated under 3 classifications:
Class A: Flame spread index 0‐25
Class B: Flame spread index 26‐75
Class C: Flame spread index 76‐200
Materials such as stone, glass and ceramic tile are rated under 25 (CLASS A).