Marble consists of recrystallized calcite
(from limestone) or dolomite
(from dolomite rock). In this hand specimen of Vermont marble, the crystals are small. For fine marble of the sort used in buildings and sculpture, the crystals are even smaller. The color of marble can range from the purest white to black, ranging through the warmer colors in between depending on the other mineral impurities.
Like other metamorphic rocks, marble has no fossils, and any layering that appears in it probably does not correspond to the original bedding of the precursor limestone. And like limestone, marble tends to dissolve in acidic fluids. It is quite durable in dry climates, as in the Mediterranean countries where ancient marble structures survive.
Commercial stone dealers use different rules than geologists to distinguish limestone from marble.
For more photos see the Metamorphic Rocks Gallery.
Geologic Features and Processes
Glaciers and Ice
Geology and Society