LAPIS LAZULI, a mineral possessing usually a fine blue colour, whence it is much prized for decorative purposes. From the large number of Egyptian ornaments in this material, which have been preserved from the time of the Pharaohs, it is evident that it was a favourite stone with the ancient Egyptians. A few Assyrian seal-cylinders in lapis lazuli are also known. It appears to have been the Greek sapphire; thus, Theophrastus describes the σα-rrφeLpοs as sprinkled with gold-dust, a description which is utterly inappropriate to any variety of our modern sapphire, but which applies with much force to the lapis lazuli, inasmuch as this stone frequently contains disseminated particles of iron pyrites, which by their colour and lustre may readily be mistaken for such a metal. Iu like manner, Pliny refers to one variety of the sapphims as being spotted with gold. An allusion to the same quality is perhaps found in Job xxviii. 6.
It is but rarely that the lapis lazuli exhibits anything approaching to distinct crystallization. Usually it occurs in compact masses, which break with an uneven fracture; but occasionally it presents an imperfect cleavage parallel to the faces of a rhombic dodecahedron, and still more rarely offers distinct faces of this form. Its specific gravity is about 2'5, and its degree of hardness between 5 and 6; it therefore scratches glass, and is capable of receiving a fair polish. Although the colour is generally a fine azure, or rich Berlin blue, some varieties exhibit violet, green, or even red tints, or are altogether colourless. The mineral is always opaque, with only slight translucency at the edges. Microscopic sections reveal a want of homogeneity in the constitution of the substance, bluish particles being disseminated through a white matrix....
The lapis lazuli is a silicate of aluminium, calcium, and sodium; but the published analyses are rather discordant. All agree, however, in recording the presence of sulphur, and it is generally supposed that this element exists as a sulphide of iron and sodium, and that it is upon the presence of such a compound that the blue colour depends. The following is an analysis of the South American variety:—silica, 45’7; alumina, 25'34; soda, 10'55; potash, 1'35; lime, 7’48; ferric oxide, l - 30,∙ sulphur, 3’96; and sulphuric acid, 4'32. By the action of hydrochloric acid the mineral is decomposed, with separation of gelatinous silica and evolution of sul- }>huretted hydrogen. Before the blowpipe it fuses readily, with oss of colour.
The lapis lazuli is usually found in crystalline limestone or in gneissose rocks, but its occurrence is confined to very limited localities. It is found in Persia, Tartary, Tibet, and China, and in the neighbourhood of Lake Baikal in Siberia. Fine masses occur in the Andes of Chili and Pera. In Europe it has been found at Ditro in Transylvania, and in the ejected blocks of Monte Somma.
In addition to its use as an ornamental stone, the lapis lazuli was formerly employed, to a large extent, in the preparation of the beautiful blue pigment called ultramarine. For this purpose the mineral was ground, calcined, and carefully levigated in water. Of late years, however, artificial ultramarine has been prepared which claims to be of equal beauty and permanency with the natural pigment. Artists, however, still regard the natural colour as superior.
