Section 14 of “A Homeowner’s Guide to Natural Stone Countertop Installation” published by The Marble Institute of America
Fissures occur naturally in many stone types. A fissure is defined by the American Geological Institute as “an extensive crack, break, or fracture in the rock, which may contain mineral-bearing material.” The term “fissure” is used commercially in the stone industry to describe a visible separation along intercrystalline boundaries or the voids between mineral crystals. This separation may start and stop within the field of the stone or extend through an edge. A fissure differs from a crack in that it is a naturally occurring feature in the stone.
Cracks occur in stones as a result of mechanically induced stresses during handling, fabrication, transport, or installation. When cracks are detected in slab material prior to fabrication, the best method is to simply avoid them during the layout process. In stones with lesser soundness properties, this option may not be possible. When working with such stones it is common practice to repair cracks by cementing them together with epoxy or polyester resin.
Chips can occur in stones either as a result of sawing operations or handling. Chips may be repaired with epoxy or polyester resin if the completed repair is consistent in color and texture with unrepaired areas of the slab.
Pitting of the countertop surface, particularly in granite, is a common characteristic of natural stones. Granites are made up of several different minerals, each mineral having a different hardness. Granites contain quartz, feldspars, biotite, amphibole, ferrous titanium oxides, and other minerals. On the Mohs Scale, diamonds are the hardest mineral, with a rating of 10. Quartz and feldspar have a hardness of 6.5 to 7 and are very durable. Biotite (mica) is very soft (2.5) and flakes easily. All true granites have biotite in their composition. Because biotite is soft and flaky, the first few layers are removed during the polishing process, causing pits throughout the slab. Some granites have more biotite throughout their composition than others. The higher the biotite content of the stone, the more pits it will have. Most polished igneous rocks will have varying degrees of pits, depending on the amount of biotite, muscovite, and phlogopite in their composition. The pits do not make the granite less durable or otherwise inferior. Pits are common in all granites and should be expected.