Marble consists of a mosaic of calcite grains that rarely show any crystalline form under the microscope, they are traversed by minute cracks, or in more severely deformed rocks, the grains show stripes which may be elongated in one particular direction they often occur interspersed with other rock types. Statuary marble achieves its luster from its ability to transmit and reflect light, which penetrates to a depth of between half and one and a half inches into the marble surface from where it is then reflected at the surfaces of deeper lying crystals. The different character and coloration of marbles from different regions is due to a variety of factors. Impurities may have been present in the original limestone which reacted during metamorphism to produce new compounds. If present in large quantities these may color the marble, green, brown back or gray. Bands of calcium silicate rock alternating with marble or forming patches or nodules can produce interesting decorative effects, but this type of rock is hard to finish because of the great difference in hardness between the silicates and carbonate materials.