When building a natural stone wall, a lime based mortar should always be used. The lime has a degree of elasticity, as well porosity, enabling the stonework to breathe and adjust with the changes of temperature and moisture. Furthermore, the lime is soft, and so critically does not grip the building stone. The lime based mortar, when mixed with yellow sand and stone dust, provides a gentle natural colour to the stone wall, enhancing it.
Straight cement mortar on the other hand is non porous, is hard and brittle. Therefore over time it retains moisture, and will crack as the building minutely adjusts over time. The cracked cement is then difficult to remove, and ultimately could damage the building stone.