When sourcing white granite for staircases, prioritize material density and water absorption rates. High-quality granite should have a water absorption rate of less than 0.4% to prevent staining and frost damage. For safety, the surface finish is critical; flamed, bush-hammered, or sandblasted finishes are essential for outdoor stairs to provide anti-slip properties (R11 or R12 rating), while polished or honed finishes are suitable for indoor use but may require anti-slip grooves or brass inserts.