Marble has been used throughout the centuries for applications both interior and exterior, both decorative and structural. From the columns of the Parthenon to Michelangelo’s statue of David, marble is universally known for its timeless beauty and endurance.
Marble is a metamorphic rock originating from the re-crystallization of limestone. Commercially, all natural calcareous rocks that are capable of taking a polish are called marbles.
Marble Soundness Classification
According to the Marble Institute of America*, marble is classified into four groups: A, B, C, and D based on the level of hardness and working qualities. The groupings should be taken into account when using marble because not all marbles are suitable for all building applications. This is particularly true for the comparatively fragile marbles in groups C and D, which may require additional fabrication before or at the time of installation.
These four groups are:
Group A: Sound marbles with uniform and favorable working qualities, containing no obvious geological fissures or voids. These stones can be used on both the interior and exterior and do not require any fillings of patching. Thassos White Marble is an example of Group A marble.
Group B: Marbles similar in character to Group A, but with less favorable working qualities. They may contain holes or voids. A limited amount of washing, sticking, and filling may be necessary and should not be highly noticeable. They are suitable for interior and exterior uses.
Group C: This group comprises the largest and most colorful group of marbles. Holes, voids, veins, and lines of separation should be expected, as well as variations in working qualities. It is standard practice to repair the variations by one or more of the following methods: washing, sticking, filling or cementing. Liners and other forms of reinforcement are used when needed. These repairs can be visible and reflect light differently. These marbles are generally suited for interior use only.
Group D: These marbles similar to Group C but contain a larger number of natural faults and maximum variations in working qualities and require more of the same methods of finishing. This group consists of the highly colored marbles prized for their decorative values.
The soundless classifications indicate what method and amount of repair and fabrication are needed before or during installation, based on standard trade practices.
*Marble Institute of America: Technical Bulletin, Vol. II, Issue 1, January 2005
Marble Color Variations:
Marble can be categorized as follows based on the uniformity of color and degree of veining.
Limited Color Variation: This refers to marble that is relatively uniform in background, veining and/or movement.
Moderate Color Variation: In these marbles, the background color has some variation and some veining; movement may appear. Inspection prior to installation is suggested.
Extreme Color Variation: The background color of these marbles has significant variations and contrast. Veining variation, as well as movement, may be inconsistent. Inspection prior to installation is strongly suggested.