CORUNDUM
(_Sapphire_, _Ruby_)
Ranking in hardness second to diamond alone, the species known to
science as corundum and widely familiar by the names of its varieties,
sapphire and ruby, holds a pre-eminent position among coloured
gem-stones. The barbaric splendour of ruby (Plate I, Fig. 13) and the
glorious hue of sapphire (Plate I, Fig. 11) are unsurpassed, and it
is remarkable that the same species should boast such different, but
equally magnificent, tints. They, however, by no means exhaust the
resources of this variegated species. Fine yellow stones (Plate I,
Fig. 12), which compare with topaz in colour and are its superior in
hardness, and brilliant colourless stones, which are unfortunately
deficient in ‘fire’ and cannot therefore approach diamond, are to
be met with, besides others of less attractive hues, purple, and
yellowish, bluish, and other shades of green. Want of homogeneity in
the coloration of corundum is a frequent phenomenon; thus, the purple
stones on close examination are found to be composed of alternate blue
and red layers, and stones showing patches of yellow and blue colour
are common. Owing to the peculiarity of their interior arrangement
certain stones display when cut _en cabochon_ a vivid six-rayed star of
light (Plate I, Fig. 15). Sapphire and ruby share with diamond, pearl,
and emerald the first rank in jewellery. They are popular stones,
especially in rings; their comparative rarity in large sizes, apart
from the question of expense, prevents their use in the bigger articles
of jewellery. The front of the stones is usually brilliant-cut and the
back step-cut, but Indian lapidaries often prefer to cover the stone
with a large number of triangular facets, especially if the stone be
flawed; star-stones are cut more or less steeply _en cabochon_.
In composition corundum is alumina, oxide of aluminium, corresponding
to the formula Al_{2}O_{3}, but it usually contains in addition small
quantities, rarely more than 1 per cent., of ferric oxide, chromic
oxide, and perhaps other metallic oxides. When pure, it is colourless;
the splendid tints which are its glory have their origin in the minute
traces of the other oxides present. No doubt chromic oxide is the
cause of the ruddy hue of ruby, since it is possible, as explained
above (p. 117), closely to imitate the ruby tint by this means, but
nothing approaching so large a percentage as 2½ has been detected in
a natural stone. The blue colour of sapphire may be due to titanic
oxide, and ferric oxide may be responsible for the yellow hue of
the ‘oriental topaz,’ as the yellow corundum is termed. Sapphires,
when of considerable size, are rarely uniform in tint throughout the
stone. Alternations of blue and red zones, giving rise to an apparent
purple or violet tint, and the conjunction of patches of blue and
yellow are common. Perfectly colourless stones are less common, a
slight bluish tinge being usually noticeable, but they are not in much
demand because, on account of their lack of ‘fire,’ they are of little
interest when cut. The tint of the red stones varies considerably in
depth; jewellers term them, when pale, pink sapphires, but, of course,
no sharp distinction can be drawn between them and rubies. The most
highly prized tint is the so-called pigeon’s blood, a shade of red
slightly inclined to purple. The prices for ruby of good colour run
from about 25s. a carat for small stones to between £60 and £80 a
carat for large stones, and still higher for exceptional rubies. The
taste in sapphires has changed of recent times. Formerly the deep blue
was most in demand, but now the lighter shade, that resembling the
colour of corn-flower, is preferred, because it retains a good colour
in artificial light. Large sapphires are more plentiful than large
rubies, and prices run lower; even for large perfect stones the rate
does not exceed £30 a carat. Large and uniform ‘oriental topazes’ are
comparatively common, and realize moderate prices, about 2s. to 30s.
a carat according to quality and size. Green sapphires are abundant
from Australia, but their tint, a kind of deep sage-green, is not very
pleasing. Brown stones with a silkiness of structure are also known.
The name of the species comes through the French _corindon_ from an
old Hindu word, _korund_, of unknown significance, and arose from the
circumstance that the stones which first found their way to Europe came
from India. At the present day the word corundum is applied in commerce
to the opaque stones used for abrasive purposes, to distinguish the
purer material from emery, which is corundum mixed with magnetite and
other heavy stones of lower hardness. The origin of the word sapphire,
which means blue, has been discussed in an earlier chapter (p. 110).
Jewellers use it in a general sense for all corundum except ruby. Ruby
comes from the Latin _ruber_, red. The prefix ‘oriental’ (p. 111)
is often used to distinguish varieties of corundum, since it is the
hardest of ordinary coloured stones and the finest gem-stones in early
days reached Europe by way of the East.
Corundum crystallizes either in six-sided prisms terminated by flat
faces (Plate I, Fig. 10), which are often triangularly marked, or with
twelve inclined faces, six above and six below, meeting in a girdle
(Plate I, Fig. 14). Ruby favours the former and the other varieties
the latter type. A fine crystal of ruby—the ‘Edwardes,’ so named by
the donor, John Ruskin, after Sir Herbert Edwardes—which weighs 33·5
grams (163 carats), is exhibited in the Mineral Gallery of the British
Museum (Natural History), and is tilted in such a way that the light
from a neighbouring window falls on the large basal face, and reveals
the interesting markings that nature has engraved on it. From its type
of symmetry corundum is doubly refractive with a direction of single
refraction running parallel to the edge of the prism. Owing to the
relative purity of the chemical composition the refractive indices
are very constant; the ordinary index ranges from 1·766 to 1·774 and
the extraordinary index from 1·757 to 1·765, the double refraction
remaining always the same, 0·009. The amount of colour-dispersion is
small, and therefore colourless corundum displays very little ‘fire.’
The difference between the indices for red and blue light is, however,
sufficiently great that the base of a ruby may be left relatively
thicker than that of a sapphire to secure an equally satisfactory
effect (cf. p. 98)—a point of some importance to the lapidary, since
stones are sold by weight and it is his object to keep the weight as
great as possible. When a corundum is tested on the refractometer in
white light a wide spectrum deliminates the two portions of the field
because of the smallness of the colour-dispersion (cf p. 25). The
dichroism of both ruby and sapphire is marked, the twin colours given
by the former being red and purplish-red, and by the latter blue and
yellowish-blue, the second colour in each instance corresponding to
the extraordinary ray. Tests with the dichroscope easily separate ruby
and sapphire from any other red or blue stone. This character has an
important bearing on the proper mode of cutting the stones. The ugly
yellowish tint given by the extraordinary ray of sapphire should be
avoided by cutting the stone with its table-facet at right angles to
the prism edge, which is the direction of single refraction. Whether
a ruby should be treated in the same way is a moot point. No doubt
if the colour is deep, it is the best plan, because the amount of
absorption of light is thereby sensibly reduced, but otherwise the
delightful nuances distinguishing ruby are best secured by cutting
the table-facet parallel to the direction of single refraction.
Yellow corundum also shows distinct dichroism, but by a variation
more of the depth than of the tint of the colour; the phenomenon is
faint compared with the dichroic effect of a yellow chrysoberyl. The
specific gravity also is very constant, varying only from 3·95 to 4·10;
sapphire is on the whole lighter than ruby. Corundum has the symbol 9
on Mohs’s scale, but though coming next to diamond it is a very poor
second (cf. p. 79). As is usually the case, the application of heat
tends to lighten the colour of the stones: those of a pale violet or
a yellow colour lose the tint entirely, and the deep violet stones
turn a lovely rose colour. On the other hand the action of radium has,
as was shown by Bordas, an intensifying action on the colour, and
even develops it in a colourless stone. From the latter reaction it
may be inferred that often in an apparently colourless stone two or
more selective influences are at work which ordinarily neutralize one
another, but, being unequally stimulated by the action of radium, they
thereupon give rise to colour. The stellate appearance of asterias
or star-stones—star-ruby and star-sapphire—results from the regular
arrangement either of numerous small channels or of twin-lamellæ in the
stone parallel to the six sides of the prisms; light is reflected from
the interior in the form of a six-rayed star (p. 38). Some stones from
Siam possess a markedly fibrous or silky structure.
The synthetical manufacture of ruby, sapphire, and other varieties of
corundum has already been described (p. 116).
Besides its use in jewellery corundum is on account of its hardness of
great service for many other purposes. Small fragments are extensively
employed for the bearing parts of the movements of watches, and both
the opaque corundum and the impure kind known as emery are in general
use for grinding and polishing softer stones, and steel and other
metal-work.
The world’s supply of fine rubies is drawn almost entirely from the
famous ruby mines near Mogok, situated about 90 miles (145 km.) in
a north-easterly direction from Mandalay in Upper Burma and at an
elevation of about 4000 ft. (1200 m.) above sea-level. It is from this
district that the stones of the coveted carmine-red, the so-called
‘pigeon’s blood,’ colour are obtained. The ruby occurs in a granular
limestone or calcite in association with the spinel of nearly the
same appearance—the ‘balas-ruby,’ oriental topaz (yellow corundum),
tourmaline, and occasionally sapphire. Some stones are found in
the limestone on the sides of the hills, but by far the largest
quantity occur in the alluvial deposits, both gravel and clay, in the
river-beds; the ruby ground is locally known as ‘_byon_.’ The stones
are as a rule quite small, averaging only about four to the carat.
Before the British annexation of the country in 1885 the mines were a
monopoly of the Burmese sovereigns and were worked solely under royal
licence. They are known to be of great antiquity, but otherwise their
early history is a mystery. It is said that an astute king secured the
priceless territory in 1597 from the neighbouring Chinese Shans in
exchange for a small and unimportant town on the Irrawaddy; if that
be so, he struck an excellent bargain. The mines were allotted to
licensed miners, _twin-tsas_ (eaters of the mine) as they were called
in the language of the country, who not only paid for the privilege,
but were compelled to hand over to the king all stones above a certain
weight. As might be anticipated this injunction caused considerable
trouble, and the royal monopolists constantly suspected the miners of
evading the regulation by breaking up stones of exceptional size; from
subsequent experience, it is probable that large stones were in reality
seldom found. Since 1887 the mines have been worked by arrangement with
the Government of India by the Ruby Mines, Ltd., an English company.
Its career has been far from prosperous, but during recent years, in
consequence of the improved methods of working the mines and of the
more generous terms afterwards accorded by the Government, greater
success has been experienced; the future is, however, to some extent
clouded by the advent of the synthetical stone, which has even made its
way out to the East.
Large rubies are far from common, and such as were discovered in the
old days were jealously hoarded by the Burmese sovereigns. According to
Streeter the finest that ever came to Europe were a pair brought over
in 1875, at a time when the Burmese king was pressed for money. One,
rich in colour, was originally cushion-shaped and weighed 37 carats;
the other was a blunt drop in form and weighed 47 carats. Both were cut
in London, the former being reduced to 32-5/16 carats and the latter
to 38-9/16 carats, and were sold for £10,000 and £20,000 respectively.
A colossal stone, weighing 400 carats, is reported to have been found
in Burma; it was broken into three pieces, of which two were cut and
resulted in stones weighing 70 and 45 carats respectively, and the
third was sold uncut in Calcutta for 7 lakhs of rupees (£46,667). The
finder of another large stone broke it into two parts, which after
cutting weighed 98 and 74 carats respectively; he attempted in vain to
evade the royal acquisitiveness, by giving up the larger stone to the
king and concealing the other. A fine stone, known by the formidable
appellation of ‘Gnaga Boh’ (Dragon Lord), weighed 44 carats in the
rough and 20 carats after cutting. Since the mines were taken over
by the Ruby Mines, Ltd., a few large stones have been discovered. A
beautiful ruby was found in the Tagoungnandaing Valley, and weighed
18½ carats in the rough and 11 carats after cutting; perfectly clear
and of splendid colour, it was sold for £7000, but is now valued at
£10,000. Another, weighing 77 carats in the rough, was found in 1899,
and was sold in India in 1904 for 4 lakhs of rupees (£26,667). A stone,
weighing 49 carats, was discovered in 1887, and an enormous one,
weighing as much as 304 carats, in 1890.
The ruby, as large as a pigeon’s egg, which is amongst the Russian
regalia was presented in 1777 to the Czarina Catherine by Gustav III of
Sweden when on a visit to St. Petersburg. The large red stone in the
English regalia which was supposed to be a ruby is a spinel (cf p. 206).
Comparatively uncommon as sapphires are in the Burma mines a faultless
stone, weighing as much as 79½ carats, has been discovered there.
Good rubies, mostly darker in colour than the Burmese stones, are
found in considerable quantity near Bangkok in Siam, Chantabun
being the centre of the trade, where, just as in Burma, they are
intimately associated with the red spinel. Because of the difference
in tint and the consequent difference in price, jewellers draw a
distinction between Burma and Siam rubies; but that, of course, does
not signify any specific difference between them. Siam is, however,
most distinguished as the original home of splendid sapphires. The
district of Bo Pie Rin in Battambang produces, indeed, more than
half the world’s supply of sapphires. In the Hills of Precious
Stones, such being the meaning of the native name for the locality, a
number of green corundums are found. Siam also produces brown stones
characterized by a peculiar silkiness of structure. Rubies are found in
Afghanistan at the Amir’s mines near Kabul and also to the north of the
lapis lazuli mines in Badakshan.
The conditions in Ceylon are precisely the converse of those obtaining
in Burma; sapphire is plentiful and ruby rare in the island. They are
found in different rocks, sapphire occurring with garnet in gneiss, and
ruby accompanying spinel in limestone, but they come together in the
resulting gravels, the principal locality being the gem-district near
Ratnapura in the south of the island. The largest uncut ruby discovered
in Ceylon weighed 42½ carats; it had, however, a decided tinge of
blue in it. Ceylon is also noted for the magnificent yellow corundum,
‘oriental topaz,’ or, as it is locally called, ‘king topaz,’ which it
produces.
Beautiful sapphires occur in various parts of India, but particularly
in the Zanskar range of the north-western Himalayas in the state of
Kashmir, where they are associated with brown tourmaline. Probably most
of the large sapphires known have emanated from India. By far the most
gigantic ever reported is one, weighing 951 carats, said to have been
seen in 1827 in the treasury of the King of Ava. The collection at the
Jardin des Plantes contains two splendid rough specimens; one, known as
the ‘Rospoli,’ is quite flawless and weighs 132-1/16 carats, and the
other is 2 inches in length and 1½ inches in thickness. The Duke of
Devonshire possesses a fine cut stone, weighing 100 carats, which is
brilliant-cut above and step-cut below the girdle. An image of Buddha,
which is cut out of a single sapphire, is exhibited, mounted on a gold
pin, in the Mineral Gallery of the British Museum (Natural History).
For some years past a large quantity of sapphires have come into the
market from Montana, U.S.A., especially from the gem-district about
twelve miles west of Helena. The commonest colour is a bluish green,
generally pale, but blue, green, yellow and occasionally red stones are
also found; they are characterized by their almost metallic lustre.
With them are associated gold, colourless topaz, kyanite, and a
beautiful red garnet which is found in grains and usually mistaken for
ruby. Rubies are also found in limestone at Cowee Creek, North Carolina.
Blue and red corundum, of rather poor quality, has come from the
Sanarka River, near Troitsk, and from Miask, in the Government of
Orenburg, Russia, and similar stones have been known at Campolongo, St.
Gothard, Switzerland.
The prolific gem-district near Anakie, Queensland, supplies examples of
every known variety of corundum except ruby; blue, green, yellow, and
parti-coloured stones, and also star-stones, are plentiful. Leaf-green
corundum is known father south, in Victoria. The Australian sapphire is
too dark to be of much value.
Small rubies and sapphires are found in the gem-gravels near the
Somabula Forest, Rhodesia.