Mineral material with stone-specific porosity, salts and cleaning sensitivities.
What changes first
Look for salt bloom, staining and surface etching. The pattern and location usually matter more than color alone.
Moisture and movement behavior
Record whether the material changes after rain, water use, heating cycles or seasonal humidity. A material can display the damage while the source is somewhere else.
Before repair
Determine whether the material is the source, a pathway or only the final surface. Correct the controlling condition before replacing finishes.