Bench

Bench : In the quarry, every natural foundation of stone limited by two consecutive "quarry beds". The quality of the material is determined by the bench (royal bench, binding bench, shell bench, etc.).

With a general sense, the deeper the mine, the better the quality of the marble.  Of course there are rifts, and upheavals that determine the marbles quality as well.   But the shallow quarries are hard to get consistent product.  You want to avoid product coming from the shell bench.