The stone we used at the Silva house is a metamorphic quartzite from a quarry in western North Carolina. It is a handsome stone, dark gray when dry and a rich black when wet. As is typical of any flat wall stone, it lays up fairly quickly. But the pieces weren't big enough to span the width of the base and bear the load of the finished wall. To get the necessary thickness and strength, we laid up a supporting wall of brick and concrete block just behind the face row of rock.
Metamorphic quartzite is attractive, stackable and harder than granite.
I then set up two yellow mason's lines, one representing the bottom of the wall, the other representing the top. On top of the base, just below the finished height of the driveway, we set our biggest stones into wet mortar. We built the Silvas' retaining wall to a width of about 28 inches, tapering to 14 inches at the cap. You always want your base twice as wide as your cap. A traditional New England stone wall is dry-stacked without mortar.