The lapis-lazuli, sometimes called "azure stone," is almost always blue, though often containing streaks of white and gold colour, the latter of which are due to the presence of minute specks or veins of iron pyrites, the former and colourless streaks being due to free lime, calcite, and other substances which have become more or less blended with the blue colour of the stone. It has a vitreous lustre, crystallises in the 1st, or cubic system, and is a complex substance, varying considerably in its ingredients in accordance with the locality in which it is found, its matrix, and the general geological formation of the surrounding substances, which may, by the penetration of moisture, be brought to bear upon the stone, thus influencing to a great extent its chemical composition. So that we find the stone composed of about a quarter of its substance of alumina, or oxide of aluminium, silica to the extent of almost half, the remainder being lime, soda, sulphur, and occasionally traces of copper and iron. It is mostly found in granite and certain crystalline limestone rocks, in fairly large masses. It is of great antiquity, figuring extensively in ancient Egyptian history, both in its form as a stone and ground up into a pigment for the decoration of sacred and royal vessels and appointments. When so ground, it forms the stable and magnificent colour, genuine ultramarine, which is the finest and purest blue on the artist's palette, but owing to its extremely high price its use is not in very great demand, especially as many excellent chemical substitutes of equal permanence are obtainable at little cost.
The Turquoise.
The turquoise is a pseudomorph (see Chapter IV., "Cleavage.") In colour it is blue or greenish-blue, sometimes opaque, varying between that and feeble translucency, though it should be said that in all forms, even those considered opaque, a thin cutting of the stone appears almost transparent, so that the usual classing of it among the opaque stones must be done with this reservation. In composition it contains about 20 per cent. of water, about a third of its substance being phosphoric acid, or phosphorus-pentoxide; sometimes nearly half of it is alumina, with small quantities of iron in the form of variously coloured oxides, with oxide of manganese. The great proportion of water, which it seems to take up during formation, is mostly obtained in the cavities of weathered and moisture-decomposing rocks. Its average formula may be said to be Al2O3P2O5 + 5H2O, and sometimes Al2O3 FeOP2O5 + 5H2O. It must therefore follow that when the stone is heated, this water will separate and be given off in steam, which is found to be the case. The water comes off rapidly, the colour of the stone altering meanwhile from its blue or blue-green to brown. If the heat is continued sufficiently long, this brown will deepen to black, while the flame is turned green. This is one of the tests for turquoise, but as the stone is destroyed in the process, the experiment should be made on a splinter from it.
This stone is of very ancient origin, and many old turquoise deposits, now empty, have been discovered in various places. History records a magnificent turquoise being offered in Russia for about £800 a few centuries ago, which is a very high price for these comparatively common stones.
Owing to the presence of phosphorus in bones, it is not uncommon to find, in certain caves which have been the resort of wild animals, or into which animals have fallen, that bones in time become subjected to the oozing and moisture of their surroundings; alumina, as well as the oxides of copper, manganese and iron, are often washed across and over these bones lying on the cave floor, so that in time, this silt acts on the substance of the bones, forming a variety of turquoise of exactly the same composition as that just described, and of the same colour. So that around the bones there eventually appears a beautiful turquoise casing; the bone centre is also coloured like its casing, though not entirely losing its bony characteristics, so that it really forms a kind of ossified turquoise, surrounded by real turquoise, and this is called the "bone turquoise" or "odontolite."
INDEX
Adamantine lustre, [28]
glimmering, [29]
glinting, or glistening, [29]
lustreless, [29]
shining, [29]
splendent, [29]
Agate, [11]
Almandine, [101]
Amethyst, [11]
oriental, [85]
sapphire, [85]
Amorphous stones and their characteristics, [23]
Analysis, [5]
Aplome, [101]
Asters, or asteriated stones, [82], [87]-91
Azure-stone, [103]
Beryl, [10], [94]
colours of, in dichroscope, [34]
Beryllium, [10]
Bezils, [66]
Black stones, list of, [79]
Blue sapphire, composition of the, [10]
stones, list of, [77]
Bone-turquoise, [106]
Break, as opposed to cleavage, [19]
Brilliant-cut stones, [66]
Brown stones, list of, [76]
Building up of crystals, [13]
Burnt, or pinked topaz, [92]
Cabochon-cut stones, [64]
(the double), [65]
(the hollow), [65]
Carbonate series, [11]
Carbuncle, [102], [103]
Cat of Egypt, [89]
Cat's eye stones, [82], [87]-91
list of (see "Chatoyant Stones"), [78]
Ceylonese cat's eye (see "Cat's eye")
Change of colour (not to be confused with "Play of colour" and "Opalescence," which see; see also "Fire"), [36]
Characteristics of precious stones, [1], [3]
Chatoyant stones, list of, [78]
Chemical illustration of formation of precious stones, [8]
Chloride of palladium in dichroscope, [34]
Chrysoberyl, [88]
Chrysolite, [11]
ordinary, or "noble", [85]
oriental, [85]
Cinnamon stone, [102]
Claims of precious stones, [4]
Cleavage affecting tests, [43]
and "cleavage" as opposed to "break", [19], [22]
Colour, [26], [28], [30], [32]
Colourless stones, list of, [75]
Colours and characteristics of the various opals, [35], [36]
of precious stones, list of, [75]-79
Common garnet, [101]
opal, [35]
Composite crystals, [13]
Composition of paste, or strass, for imitation stones, [71]
Composition of precious stones, [7]
Converted stones, [72]
Corundum, [82]
Crown portion of stones, [65], [66]
Crystalline structure, physical properties, of [13]
Crystallography, [14]
Crystals, axes of symmetry, [15]
groups of, [15], [16]
planes of symmetry, [15]
systems of, [16]
(1) Cubic—isometric, monometric, regular, [16]
(2) Hexagonal—rhombohedral, [16]
(3) Tetragonal—quadratic, square prismatic, dimetric, pyramidal, [16]
(4) Rhombic—orthorhombic, prismatic, trimetric, [16]
(5) Monoclinic—clinorhombic, monosymmetric, oblique, [16], [17]
(6) Triclinic—anorthic, asymmetric, [16], [17]
treatment of, [14]
Culasse portion of stones, [66]
Cullinan diamond (see also "Stars of Africa"), [22], [64], [68], [80]
Cutting of precious stones, [3], [4], [62]
Cymophane, [90]
Definition of a precious stone, [1]
Diamond, characteristics of the, [80]
composition of the, [10]
(sapphire), [86]
unique, [10]
(zircon), [99]
Diaphaneity, [26], [28]
Diaphanous stones, [28]
Dichroscope, [33]
how to make a, [33]
how to use a, [34]
Dimorphism in precious stones, [25]
Double cabochon-cut stones, [65]
refraction (see "Refraction")
Doublets, [72]
Electric and magnetic influences, [57]
experiments with precious stones and pithball and electroscope, [57]
experiments with tourmaline, [58], [59]
Emerald, [10], [11], [95], [96]
oriental, [85]
En cabochon-cut stones, [64]
Experiments to show electric polarity, [58], [59]
Facets in stones, description of the, [67], [68]
Feminine stones, [85]
Fire in stones (see also "Change of Colour," "Opalescence," and "Play of Colour"), [36], [37]
Fire opal, [35]
Flame-coloured stones, list of, [76]
Flaws, [63]
Formation of precious stones, [5], [8]
chemical illustration of, [8], [9]
Garnet, [11], [100]
Garnets
(A) iron-alumina (called also almandine and precious or oriental garnet), [101]
sub-variety, common garnet, [101]
(B) lime-iron, [101]
sub-variety aplome, [101]
melanite, [101]
pyreneite, [101]
topazolite, [101]
(C) lime-chrome, [101], [102]
sub-variety uwarowite, [101], [102]
(D) lime-alumina, [102]
sub-variety cinnamon stone, [102]
romanzovite, [102]
succinite, [102]
(E) magnesia-alumina, [102], [103]
sub-variety carbuncle, or anthrax, [102], [103]
noble, [103]
pyrope, [102]
(F) manganese-alumina, [103]
sub-variety spessartine, or spessartite, [103]
Girdle portion of a stone, [66]
Glimmering, in lustre, definition of, [29]
Glinting, or glistening in lustre, definition of, [29]
Goutte de suif-cut stones, [65]
Great Mogul diamond, [64]
Green stones, list of, [78]
Groups of crystals (see "Crystals")
Hardness, physical properties of, [39]
table of, [39], [40], [41]
Heat indexes, [54]
physical properties of, [52]
Hollow-cabochon, [65]
Hyacinth, ordinary (a form of zircon), [85], [98]
oriental, [85]
Hyalite (opal), [35]
Hydrophane (opal), [35]
Imitations and tests of precious stones, [70]
Indigo sapphires, [86]
Ink sapphires, [85]
Iridescence, and cause of, [37], [38]
Iron-alumina garnets, [101]
Jacinth, oriental, [85]
Jarcon, or jargoon, [98]
Koh-i-nûr, [64]
Lapis-lazuli, [103]
Light, physical properties of, [26]
Lime-alumina garnets, [102]
cinnamon stone, [102]
romanzovite, [102]
succinite, [102]
Lime-chrome garnets, [101], [102]
uwarowite, [101], [102]
Lime-iron garnets, [101]
aplome, [101]
pyreneite, [101]
topazolite, [101]
List of stones according to colour, [75]-79
hardness, [39]-41
specific gravity, [48]-50
Lustre, [26], [28]
Lustreless, definition of, [29]
Lynx-eye stones, [87]
Magnesia-alumina garnets, [102], [103]
carbuncle, or anthrax, [102]
noble, [103]
pyrope, [102]
Magnetic and electric influences, [57]-61
Malachite, [11]
Manganese-alumina garnets, [103]
spessartine, or spessartite, [103]
Masculine stones, [85]
Melanite, [101]
Menilite (opal), [36]
Metallic-lustre stones, [28], [29]
Mohs's table of hardness, [39]-41
Noble garnet, [103]
or precious opal, [35]
Non-diaphanous stones, [28]
Odontolite, [106]
Olivine corundum (see "Chrysolite"), [85]
Opal, [11]
varieties of, [35], [36]
Opalescence (not to be confused with "Change of Colour" and "Play of Colour," which see; see also "Fire"), [36], [37]
Oriental amethyst, [85]
cat's eye (see "Cat's eye")
emerald, [85]
garnet, [101]
topaz, [85]
Origin of precious stones, [7]
Paste, or strass, for imitation stones, composition of, [71]
Pavilion portion of cut stones, [66]
Pearly-lustre stones, [28], [29]
Peridot (see "Noble Chrysolite"), [85]
Pink-coloured stones, list of (see also Red), [77]
Pinked topaz, [92]
Phosphorescence, [26], [30]
Physical properties:—
A.—Crystalline structure, [13]
B.—Cleavage, [19]
C.—Light, [26]
D.—Colour, [32]
E.—Hardness, [39]
F.—Specific gravity, [45]
G.—Heat, [52]
H.—Magnetic and electric influences, [57]
Play of colour (not to be confused with "Change of Colour" and "Opalescence," which see; see also "Fire"), [36], [37]
Pleochroism, [33]
Polarisation, electric, [58], [59]
of light, [26], [27]
Polariscope, [27], [28]
Polishing precious stones, [3], [4]
Polymorphism in precious stones, [25]
Precious, or noble opal, [35]
Pseudomorphism in precious stones, [23], [24]
Pyreneite, [101]
Pyro-electricity, development and behaviour of, [58]-60
Pyrope, [102]
Qualities of precious stones, [1], [3]
Red and rose-coloured stones, list of (see also Pink), [76], [77]
Reflection of light, [26], [28]
Refraction of heat, [52]-55
light, [26], [27]
Reproduction of crystalline form, [20], [21]
Resinous lustre stones, [28], [29]
Rock-crystal, [11]
Romanzovite, [102]
Rose-coloured stones (see Red, above), [76], [77]
Rose, or rosette-cut stones, [65]
Rothschild's testing solution, [73]
Ruby, characteristics of, [83]
composition of, [10]
Sapphire, amethyst, [85]
and its varieties, [84], [85]
cleared, [86]
diamonds, [87]
indigo, [86]
ink, [85]
the blue, composition of, [10], [85]
water, [86]
Semi-diaphanous stones, [28]
Shining, in lustre, definition of, [29]
Silica group, composition of the, [11]
Silicates, [100]
Silky-lustre stones, [28], [29]
Single-refraction (see "Refraction")
South African diamond (see "Cullinan Diamond")
Specific gravity, [45]
Splendent, in lustre, definition of, [29]
Splitting of the Cullinan diamond, [22]
Star-portion of stones, [65]
Stars of Africa (see also "Cullinan Diamond"), [22], [64], [68]
Starting or splitting of stones on cleavage planes, [23]
Step-cut stones, [66]
Stones arranged according to colour, [75]-79
hardness, [39]-41
specific gravity, [48]-50
Strass for imitation stones, composition of, [71]
Sub-metallic in lustre, definition of, [29]
Sub-translucent stones, [28]
Sub-transparent stones, [28]
Succinite, [102]
Synthesis, [5]
Systems of crystals (see "Crystals")
Table-cut stones, [65]
Tallow drops, [65]
Teeth of stone, [65]
Testing by crystalline structure, [17]
hardness, [40], [43]
with needles, [41]
gems by dichroscope, [33], [34]
solution (Rothschild's), [73]
Tests of precious stones (general), [70]
Topaz, [11], [91]
colours of, in dichroscope, [34]
oriental, [85]
Topazolite, [101]
Tourmaline, [96], [97]
electric experiments with, [58], [59]
Translucent stones, [28]
Transmission of heat, [52]-56
light, [26]
Transparent stones, [28]
Trap-cut stones, [66]
Tri-morphism in precious stones, [25]
Triplets, [72]
Turquoise, [104]
(bone), [106]
composition of the, [11]
odontolite, [106]
Uwarowite, [101], [102]
Violet stones, list of, [78]
Vitreous-lustre stones, [28], [29]
Water-sapphires, [86]
White (paste) stones, [71]
stones, list of, [75]
Yellow stones, list of, [76]
topaz, [92]
Zircon, [10], [98]
diamonds, [99]
Zirconium, [10]