Marble and its cousins, onyx, travertine, limestone and Jerusalem stone, is a sedimentary rock. This is formed under pressure by a combination of materials such as silt, plant deterius, animal skeletons and sea shells that accumulate and solidify under bodies of water over millions and millions of years. Its main component is calcium which is much softer than the materials found in granite. It is quarried and cut in much the same way as granite. It is graded on a scale from A to D which does not refer to its rarity, but rather to its content or lack of content of natural fissures and veining. The group A has the least amount of fissures while group D has the most. Group C and D marbles will generally be reinforced on the back of the slab with a fiberglass mesh screen embedded in a coat of resin. This gives the slab greater structural integrity than it would otherwise possess. During the fabrication process, many marble pieces are further reinforced with metal rods to give them added strength.