[1] _The Universal Magazine of Knowledge and Pleasure_ for 1749
states that diamond dust, "well ground and diluted with water and
vinegar, is used in the sawing of diamonds, which is done with an
iron or brass wire, as fine as a hair."--Ed.
LAPILLI (pl. of Ital. _lapillo_, from Lat. _lapillus_, dim. of _lapis_,
a stone), a name applied to small fragments of lava ejected from a
volcano. They are generally subangular in shape and vesicular in
structure, varying in size from a pea to a walnut. In the Neapolitan
dialect the word becomes _rapilli_--a form sometimes used by English
writers on volcanoes. (See VOLCANOES.)
LAPIS LAZULI, or azure stone,[1] a mineral substance valued for
decorative purposes in consequence of the fine blue colour which it
usually presents. It appears to have been the sapphire of ancient
writers: thus Theophrastus describes the [Greek: sappheiros] as being
spotted with gold-dust, a description quite inappropriate to modern
sapphire, but fully applicable to lapis lazuli, for this stone
frequently contains disseminated particles of iron-pyrites of gold-like
appearance. Pliny, too, refers to the _sapphirus_ as a stone sprinkled
with specks of gold; and possibly an allusion to the same character may
be found in Job xxviii. 6. The Hebrew _sappir_, denoting a stone in the
High Priest's breastplate, was probably lapis lazuli, as acknowledged in
the Revised Version of the Bible. With the ancient Egyptians lapis
lazuli was a favourite stone for amulets and ornaments such as scarabs;
it was also used to a limited extent by the Assyrians and Babylonians
for cylinder seals. It has been suggested that the Egyptians obtained it
from Persia in exchange for their emeralds. When the lapis lazuli
contains pyrites, the brilliant spots in the deep blue matrix invite
comparison with the stars in the firmament. The stone seems to have been
sometimes called by ancient writers [Greek: kyanos]. It was a favourite
material with the Italians of the _Cinquecento_ for vases, small busts
and other ornaments. Magnificent examples of the decorative use of lapis
lazuli are to be seen in St Petersburg, notably in the columns of St
Isaac's cathedral. The beautiful blue colour of lapis lazuli led to its
employment, when ground and levigated, as a valuable pigment known as
ultramarine (q.v.), a substance now practically displaced by a
chemical product (artificial ultramarine).
Lapis lazuli occurs usually in compact masses, with a finely granular
structure; and occasionally, but only as a great rarity, it presents
the form of the rhombic dodecahedron. Its specific gravity is 2.38 to
2.45, and its hardness about 5.5, so that being comparatively soft it
tends, when polished, to lose its lustre rather readily. The colour is
generally a fine azure or rich Berlin blue, but some varieties exhibit
green, violet and even red tints, or may be altogether colourless. The
colour is sometimes improved by heating the stone. Under artificial
illumination the dark-blue stones may appear almost black. The mineral
is opaque, with only slight translucency at thin edges.
Analyses of lapis lazuli show considerable variation in composition, and
this led long ago to doubt as to its homogeneity. This doubt was
confirmed by the microscopic studies of L. H. Fischer, F. Zirkel and H.
P. J. Vogelsang, who found that sections showed bluish particles in a
white matrix; but it was reserved for Professor W. C. Brögger and H.
Bäckström, of Christiania, to separate the several constituents and
subject them to analysis, thus demonstrating the true constitution of
lapis lazuli, and proving that it is a rock rather than a definite
mineral species. The essential part of most lapis lazuli is a blue
mineral allied to sodalite and crystallized in the cubic system, which
Brögger distinguishes as lazurite, but this is intimately associated
with a closely related mineral which has long been known as haüyne, or
haüynite. The lazurite, sometimes regarded as true lapis lazuli, is a
sulphur-bearing sodium and aluminium silicate, having the formula:
Na4(NaS3Al) Al2(SiO4)3. As the lazurite and the haüynite seem to occur
in molecular intermixture, various kinds of lapis lazuli are formed; and
it has been proposed to distinguish some of them as lazurite-lapis and
haüyne-lapis, according as one or the other mineral prevails. The
lazurite of lapis lazuli is to be carefully distinguished from lazulite,
an aluminium-magnesium phosphate, related to turquoise. In addition to
the blue cubic minerals in lapis lazuli, the following minerals have
also been found: a non-ferriferous diopside, an amphibole called, from
the Russian mineralogist, koksharovite, orthoclase, plagioclase, a
muscovite-like mica, apatite, titanite, zircon, calcite and pyrite. The
calcite seems to form in some cases a great part of the lapis; and the
pyrite, which may occur in patches, is often altered to limonite.
Lapis lazuli usually occurs in crystalline limestone, and seems to be a
product of contact metamorphism. It is recorded from Persia, Tartary,
Tibet and China, but many of the localities are vague and some doubtful.
The best known and probably the most important locality is in Badakshan.
There it occurs in limestone, in the valley of the river Kokcha, a
tributary to the Oxus, south of Firgamu. The mines were visited by Marco
Polo in 1271, by J. B. Fraser in 1825, and by Captain John Wood in
1837-1838. The rock is split by aid of fire. Three varieties of the
lapis lazuli are recognized by the miners: _nili_ of indigo-blue colour,
_asmani_ sky-blue, and _sabzi_ of green tint. Another locality for lapis
lazuli is in Siberia near the western extremity of Lake Baikal, where it
occurs in limestone at its contact with granite. Fine masses of lapis
lazuli occur in the Andes, in the vicinity of Ovalle, Chile. In Europe
lapis lazuli is found as a rarity in the peperino of Latium, near Rome,
and in the ejected blocks of Monte Somma, Vesuvius. (F. W. R.*)