A hydrous calcium sulphate, alabaster is crypto-crystalline in structure and generally white in color, although it is often streaked with reddish impurities. Browns and yellows sometimes occur; all are opaque, with a greasy luster. Nevertheless, some degree of polish may be obtained. Alabaster is, however, very soft compared with most gem stones, reaching only 1 1/2 to 2 on the scale of hardness, and it may be scratched with the finger nail. Specific gravity is about 2.3 and it is slightly soluble in water. Suitable material is cut into a variety of shapes, such as lamp shades, without much difficulty. Material found near Volterra in Italy is of a rich brown color, sometimes passing to a lemon yellow, and this is much esteemed. Florence has a considerable trade in this stone, but it is also found in Switzerland, the Tyrol, and the U.S.A. Very ancient quarries exist in Algeria, from whence it is thought that the material mentioned in the Bible originated. On the other hand, it may have been quarried in the neighborhood of Thebes (Egypt), which still produces useful stones. Alabaster was used by the ancients for making small perfume bottles. The Romans used to make vases from this material to hold the tears which widows shed on the death of their husbands; hence they were called lachrymatory vases. Also cinerary urns were made, in which ashes of the dead were preserved.
The term alabaster may be derived from the Arabic al bakstra-ton, meaning "the whitish stone." There are three other substances which somewhat resemble alabaster in chemical composition—selenite, satin spar, and gypsum. All belong to the monoclinic system of crystallography; their hardness is about 1 1/2 to 2, and specific gravities are about 2.30. Selenite (derived from a Greek word meaning moon, in allusion to its pearly luster), is a crystalline form of sulphate of lime. It may be easily split with a knife into thin, transparent layers or plates. Such plates have been used as window panes, especially in Bolivia. Satin spar is a fibrous variety of massive gypsum which is found in Derbyshire, Nottinghamshire, and in parts of Gloucestershire. It is softer than cat's-eye, which it somewhat resembles. A material often called satin spar is a white, fibrous variety of limestone with a satin-like luster, moderately hard, and sometimes carved and worked in the form of boxes and other small ornaments. It is really a carbonate of lime, a form of calcite, and it is found in veins or crevices in rocks, the fibers stretching across the crevices. The massive form of sulphate of lime is called gypsum, and this, when heated to expel the water and then ground to a powder, is the Plaster of Paris so well known in commerce. It is a white powder, granular and compact, and almost insoluble in water. When mixed with a small quantity of water so as to form a thin paste, it gradually thickens and soon solidifies into a hard mass of hydrated sulphate, the water being absorbed. On account of this property, it is of great use in taking casts and molds of objects; it is also used to some extent as a plaster, although the climate of some countries renders it unsuitable for that purpose. Gypsum is widely distributed over many countries. |
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Among the opaque minerals which we are about to describe in this chapter, alabaster is one of those which is never mounted in jewelry, its use being generally confined to building and decorative work. But like all the species which follow, it has some distinguishing features which raise it above ordinary rock.