MARBLE, a term applied to any limestone or dolomite which is sufficiently close in texture to admit of being polished (from Lat. marmor, Gr. papp.apos shining stone). Many other orna mental stones—such as serpentine, alabaster and even granite— are sometimes loosely designated marble, but by accurate writers the term is invariably restricted to those crystalline and compact varieties of carbonate of lime (occasionally with carbonate of magnesia) which, when polished, are decorative. The crystalline structure is typically shown in statuary marble. A fractured surface of this stone displays a multitude of sparkling facets, which are the rhombohedral cleavage-planes of the component grains. The beautiful lustre of polished statuary marble is due to the light penetrating for a short distance into the rock and then suffering reflection at the surfaces of the deeper-lying crystals. The durability of marble in a dry atmosphere or when protected from rain renders it a valuable building stone ; on the other hand, when exposed to the weather or the acid atmosphere of large cities, its surface readily crumbles.
Statuary and Economic Marbles.—Among statuary marbles the first place may be assigned to the famous Pentelic marble, the material in which Pheidias, Praxiteles and other Greek sculptors executed their principal works; it came from the quarries of Mount Pentelicus in Attica, and its characteristics are well seen in the Elgin marbles from the Parthenon at Athens now at the British Museum. Parian marble, another stone much used by Greek sculptors and architects, was quarried in the isle of Paros, chiefly at Mount Marpessa. It is called by ancient writers lychnites (Gr. Mxvos, a lamp) in allusion to the fact that the quarries were worked by the light of lamps. The Venus de' Medici is a notable example of work in this material. Carrara. marble is better known than any of the Greek marbles, inas much as it constitutes the stone invariably employed by the best sculptors of the present day; it occurs abundantly in the Apuan Alps, an offshoot of the Apennines, and is largely worked in the neighbourhood of Carrara, Massa and Serravezza. Stone from this district was employed in Rome for architectural pur poses in the time of Augustus, but the finer varieties, adapted to the needs of the sculptor, were not discovered until some time later. It is in Carrara marble that the finest works of
Michelangelo and of Canova are executed; the purest varieties are snow-white and are of fine saccharoidal texture. Silica is disseminated through some of the marble, becoming a source of annoyance to the workman ; while occasionally it separates as beautifully pellucid crystals of quartz known as "Carrara diamonds." Other Varieties of Marble.—Certain calcareous metamorphic rocks frequently form stones which are sufficiently beautiful to be used for ornamental purposes, and are generally classed as marbles. Such serpentinous limestones are included by petrologists under the term ophicalcite. The famous verde antico is a rock of this character.
Many marbles which are prized for the variegated patterns they display owe these patterns to their formation in concentric zones—such marbles being in fact stalagmitic deposits of carbon ate of lime, sometimes consisting of aragonite. One of the most beautiful stalagmitic rocks is the so-called onyx marble of Al geria. This was largely used in the buildings of Carthage and Rome, but the quarries which yielded it were rediscovered near Oued-Abdallah only in 1849. The stone is a beautifully translu cent material, delicately clouded with yellow and brown, and is greatly prized by French workmen. Large deposits of a very fine onyx-like marble, similar to the Algerian stone, have been worked at Tecali, about 35 m. from the city of Mexico. Among other stalagmitic marbles, mention may he made of the well known Gibraltar stone, which is often worked into models of cannon and other ornamental objects. This stalagmite is much deeper in colour and less translucent than the onyx marbles of Algeria and Mexico. A richly tinted stalagmitic stone worked in California is known as Californian marble. It is worth noting that the "alabaster" of the ancients was stalagmitic carbonate of lime, and that this stone is therefore called by mineralogists "Oriental alabaster" in order to distinguish it from our modern "alabaster," which is a sulphate, and not a carbonate, of lime. Gypsum capable of taking a polish is found at Fauld in Stafford shire and in Italy and Spain.