Powdery residue on limestone despite many washings

BED
In granites and marbles a layer or sheet of the rock mass that is horizontal, commonly curved and lenticular as developed by fractures. Sometimes applied also to the surface of parting between the sheets. Or, in stratified rocks the unit layer formed by sedimentation; of variable thickness and commonly tilted or distorted by subsequent deformation; generally develops a rock cleavage, parting or jointing along the planes of stratification

EFFLORESCENCE
A deposit or encrustation of soluble salts generally white, usually carbonates or sulfates, that may form on the surface of stone, brick, concrete or mortar when moisture moves through and evaporates from the masonry

LIMESTONE
Sedimentary rock originated mostly by the decomposition of marine organism composed primarily of calcite or dolomite. The varieties of limestone used as dimension stone are usually well consolidated and exhibit a minimum of graining or bedding direction