I have been doing masonry, and specifically stone masonry, for about 20 years of my life. I have made my share of errors over time, each one a learning experience. As I now consider myself one of the old-timers, I wish to share two critical mistakes in construction which can prevent homeowners from wasting thousands of dollars. Each year, without fail, I rebuild structures which embody these critical errors. If your contractor intends to utilize and of these techniques, do NOT hire.
Using mortar where inappropriate. I build in the New England, so some of my thoughts are not appropriate for those that do not experience our freeze and thaw cycles. Mortar has fantastic adhesive properties, meaning stones will stick to each other. On the other hand, it has very low tensile strength, meaning it will fracture with movement. Unless a structure is built upon a concrete foundation or footing, mortar should not be used outdoors.
Pea-stone as filler for stone walls. I see this technique being used quite often. It is an extremely inexpensive and easy way to fill or backfill a stone wall. The problem is you may as well be building wi5 marbles. As your stone wall moves during freeze/thaw cycles, these little stones will become lodged under the wall stones causing failure. I have included a picture to demonstrate. The proper fill is erosion stone (frequently referred as rip rip).