Water penetrating exterior wall cavities through defective flashing or unsealed joints can cause
efflorescence, a mineral salt residue left on the surface
of masonry when water evaporates. In addition, condensation in wall cavities prevented from reaching the exterior surface because of blocked weep holes can dislodge masonry in a freeze-thaw climate. Look for a darkening affect of the stone. It is recommended that you contact your stone professional for a remedy.
Moisture damage on exterior floor slabs.
Moisture coming up through a floor slab seeks the easiest possible pathway to evaporate into the atmosphere. Often, the veining or micro-cracks in the structures of some stones provide that path. The moisture dissolves all the salts from the ground, the substrate, and the stone, carries them to the surface, and deposits them as the moisture evaporates, giving the appearance of a faulty stone.
Contact your stone professional for assistance.