Marble, in its strict and proper sense, is a rock crystallised in a saccharoidal manner, having the fracture of loaf-sugar, and composed of carbonate of lime, either almost pure when the colour is white, or combined with oxide of iron or other impurities which give various colours to it. But many other kinds of stone are popularly included under this title. Indeed any limestone rock sufficiently compact to admit of a polish is called marble. It is only in this vague sense that the indurated amorphous rocks used in Britain can receive this name. Such are the black, red, gray, and variegated limestones of the Devonian system, which are very beautiful from the numbers of exquisitely-preserved corals which abound in them; the marbles of the Carboniferous series from Flintshire, Derbyshire, and Yorkshire, so full of encrinites; the shell marbles from the Oolitic rocks at Rance, Stamford, and Ycovil; and the dark Purbeck and Petworth marbles, beautifully 'figured' with shells, from the Wealden strata, which were so much used by the architects of the middle ages.
Saccharine or statuary marble is a white fine-grained rock, resembling loaf-sugar in colour and texture, working freely in every direction, not liable to splinter, and taking a fine polish. Of the marbles used by the ancients, the most famous was Parian marble, a finely granular and very durable stone, with a waxy appearance when polished. Some of the finest Grecian sculptures were formed of this marble, among others, the famous Venus de Medici. The marble of Pentelicus was at one time preferred by the Greeks to Parian, because it was whiter and finer grained. The Parthenon was entirely built of it, and many famous statues still remain which were executed in this marble, but they are always more or less weathered, never retaining the beautiful finish of the Parian statues. The quarries at Carrara (q. v.) were known to the ancients, but they have been more extensively wrought for modern sculptors, who use this marble chiefly. It is a fine-grained, pure white marble, but is so often traversed by gray veins that it is difficult to get large blocks free from these. Of coloured marbles, the best known are the Rosso Antico, a deep blood-red, sprinkled with minute white dots; Verde Antico, a clouded green produced by a mixture of white marble and green serpentine; Giallo Antico, a deep yellow, with black or yellow rings; and Nero Antico, a deep black marble.
A true marble is a crystalline granular aggregate of calcite, the granules being of remarkably uniform size. Not infrequently scales of mica or talc occur scattered through the rock. Such a rock is of metamorphic origin: it is simply a limestone which has been rendered entirely crystalline from the effects of heat under pressure, as in the vicinity of large intrusions of igneous rock. Marble may therefore be of any geological age. Many crystalline limestones, which are sometimes entitled to the name of marble, occur associated with gneiss and mica-schist, and are often rich in such minerals as garnet, actinolite, zoisite, mica, &c.