Khushūnah: Roughness, unevenness of a surface.
Malāsah: Smoothness, an even surface.
Līn: Softness or a tendency to yield under pressure.
Ṣalābah: Hardness or resistance to pressure.
These are not interchangeable. A stone may be smooth on the surface yet hard under pressure. Another substance may yield easily while remaining uneven in texture.
“As smooth as silk” selects smoothness; “as hard as stone” selects hardness. Naming only “a tactile quality” without the precise respect leaves the analysis incomplete.