I have been told that I have a class D marble, which is very brittle. What does this mean?

The Marble Institute of America has developed four classifications for marble. This classification system is based on how sound the material is. A class A marble would be the the most sound where as a class D marble is very weak. The following is a brief description of the classifications.

Class A: Sound marble with uniform and favorable working qualities. Class B: Similar in character to class A, but working qualities less favorable; occasional natural faults; limited amount of filling needed. Class C: Uncertain variation in working qualities; geological flaws, voids, veins and lines of separation common. Class D: Similar to class

CAUTION this classification system only applies to marble. Do not use this system to classify granite.