We have also seen that if an equal light-beam becomes obstructed in its passage by some substance which is denser than atmospheric air, it will become altered in its direction by refraction or reflection, and polarised, each side or pole having different properties.

Polarised light cannot be made again to pass in a certain direction through the crystal which has polarised it; nor can it again be reflected at a particular angle; so that in double-refracting crystals, these two poles, or polarised beams, are different in colour, some stones being opaque to one beam but not to the other, whilst some are opaque to both.

This curious phenomenon, with this brief, though somewhat technical explanation, shows the cause of many of the great charms in precious stones, for when viewed at one angle they appear of a definite colour, whilst at another angle they are just as decided in their colour, which is then entirely different; and as these angles change as the eye glances on various facets, the stone assumes a marvellous wealth of the most brilliant and intense colour of kaleidoscopic variety, even in a stone which may itself be absolutely clear or colourless to ordinary light.

Such an effect is called pleochroism, and crystals which show variations in their colour when viewed from different angles, or by transmitted light, are called pleochroic, or pleochromatic — from two Greek words signifying “to colour more.” To aid in the examination of this wonderfully beautiful property possessed by precious stones, a little instrument has been invented called the dichroscope, its name showing its Greek derivation, and meaning “to see colour twice” (twice, colour, to see). It is often a part of a polariscope; frequently a part also of the polarising attachment to the microscope, and is so simple and ingenious as to deserve detailed explanation.

In a small, brass tube is fixed a double-image prism of calcite or Iceland spar, which has been achromatised (that is, clear, devoid of colour) and is therefore capable of transmitting light without showing any prismatic effect, or allowing the least trace of any except the clear light-beam to pass through. At one end of this tube there is a tiny square hole, the opposite end carrying a small convex lens, of such a strength or focus as to show the square hole in true focus, that is, with perfectly sharp definition, even up to the corners of the square. On looking through the tube, the square hole is duplicated, two squares being seen.

The colors of a gem are tested by the stone being put in front of this square, when the two colours are seen quite distinctly. Not only is this a simple means of judging colour, but it enables a stone to be classified readily. For if the dichroscope shows two images of _the same_ colour, then it may possibly be a carbuncle, or a diamond, as the case may be –for single-refracting stones, of the first or cubic system, show two images of the same color. But if these two colours are different, then it must be a double-refracting stone, and according to the particular colours seen, so is the stone classified, for each stone has its own identical colour or colours when viewed through this small but useful instrument.

How clear and distinct are these changes may be viewed without it in substances strongly dichroic; for instance, if common mica is viewed in one direction, it is transparent as polished plate-glass, whilst at another angle, it is totally opaque. Chloride of palladium also is blood-red when viewed parallel to its axis, and transversely, it is a remarkably bright green. The beryl also, is sea-green one way and a beautiful blue another; the yellow chrysoberyl is brown one way and yellow with a greenish cast when viewed another way. The pink topaz shows rose-colour in one direction and yellow in another. These are perhaps the most striking examples, and are mostly self-evident to the naked eye, whilst in other cases, the changes are so delicate that the instrument must be used to give certainty; some again show changes of colour as the stone is revolved in the dichroscope, or the instrument revolved round the stone.

Some stones, such as the opal, split up the light-beams as does a prism, and show a wonderful exhibition of prismatic colour, which is technically known as a “play of colour.” The descriptive term “opalescence” is self-suggesting as to its origin, which is the “noble” or “precious” opal; this radiates brilliant and rapidly changing iridescent reflections of blue, green, yellow and red, all blending with, and coming out of, a curious silky and milky whiteness, which is altogether characteristic.

The moonstone is another example of this peculiar feature which is possessed in a more or less degree by all the stones in the class of pellucid jewels, but no stone or gem can in any way even rival the curious mixture of opaqueness, translucency, silkiness, milkiness, fire, and the steadfast changeable and prismatic brilliance of colour of the precious opal.

The other six varieties of opal are much inferior in their strange mixture of these anomalies of light and colour.

Given in order of value, we have as the second, the “fire” opal with a red reflection, and, as a rule, that only.

The third in value is the “common” opal, with the colours of green, red, white and yellow, but this is easily distinguishable from the “noble” or “precious” variety in that the common opal does not possess that wonderful “play” of colour.

The fourth variety is called the “semi-opal,” which is really like the third variety, the “common,” but of a poorer quality and more opaque.

The fifth variety in order of value, is that known as the “hydrophane,” which has an interesting characteristic in becoming transparent when immersed in water, and only then.

The sixth is the “hyalite,” which has but a glassy or vitreous lustre, and is found almost exclusively in the form of globules, or clusters of globules, somewhat after the form and size of bunches of grapes; hence the name “botryoidal” is often applied to this variety.

The last and commonest of all the seven varieties of opal is somewhat after the shape of a kidney (reniform), or other irregular shape, occasionally almost transparent, but more often somewhat translucent, and very often opaque. This seventh class is called “menilite,” being really an opaline form of quartz, originally found at Menilmontant, hence its name (Menil, and Greek lithos, stone). It is a curious blue on the exterior of the stone, brown inside.

History records many magnificent and valuable opals, not the least of which was that of Nonius, who declined to give it to Mark Antony, choosing exile rather than part with so rare a jewel, which Pliny describes as being existent in his day, and of a value which, in present English computation, would exceed one hundred thousand pounds.

Many other stones possess one or more properties of the opal, and are therefore considered more or less opalescent. This “play of colour” and “opalescence,” must not be confused with “change of colour.”

The two first appear mostly in spots and in brilliant points or flashes of coloured light, or “fire” as it is termed. This fire is constantly on the move, or “playing,” whereas “change of colour,” though not greatly dissimilar, is when the fire merely travels over broader surfaces, each colour remaining constant, such as when directly moving the stone, or turning it, when the broad mass of coloured light slowly changes, usually to its complementary. Thus in this class of stone, subject to “change of colour,” a green light is usually followed by its complementary, red, yellow by purple, blue by orange, green by brown, orange by grey, purple by broken green, with all the intermediary shades of each.

Thus when the line of sight is altered, or the stone moved, never otherwise, the colours chase one another over the surface of the gem, and mostly in broad splashes; but in those gems possessing “play of colour,” strictly speaking, whilst the stone itself remains perfectly still, and the sight is fixed unwaveringly upon it, the pulsations of the blood in the eyes, with the natural movements of the eyes and eyelids, even in a fixed, steady glance, are quite sufficient to create in the stone a display of sparks and splashes of beautiful fiery light and colour at every tremor.

The term “iridescence” is used when the display of colour is seen on the surface, rather than coming out of the stone itself. The cause of this is a natural, or in some cases an accidental, breaking of the surface of the stone into numerous cobweb-like cracks; these are often of microscopic fineness, only perceptible under moderately high powers.

Nevertheless they are quite sufficient to interfere with and refract the light rays and to split them up prismatically. In some inferior stones this same effect is caused or obtained by the application of a gentle heat, immersion in chemicals, subjection to “X rays” and other strong electric influence, and in many other ways.

As a result, the stone is very slightly expanded, and as the molecules separate, there appear on the surface thousands, perhaps millions, of microscopic fissures running at all angles, so that no matter from what position the stone may be viewed, a great number of these fissures are certain to split up the light into prismatic colours causing brilliant iridescence. Similar fissures may often be seen with the naked eye on glass, especially if scorched or cooled too rapidly (chilled), and on the surface of clear spar and mica, their effects being of extreme interest, from a colour point of view, at least.

Hardness. Specific Gravity.

Carnelian (a variety of Chalcedony) 6-1/2 2.598-2.610
Diamond 10 3.502-3.564
Deep Red Garnet 7-1/4 3.40-4.50
Jasper 7 2.668
Opal (the “Fire Opal”) 5-1/2-6-1/2 2.21
Ruby 9 4.073-4.080
Rhodonite 5-1/2-6-1/2 3.413-3.617
Sapphire 9 4.049-4.060
Spinel Ruby 8 3.614-3.654
Topaz 8 3.500-3.520
Tourmaline 7-1/4 3.024
Zircon 7-1/2 4.70-4.88

But as it cools it becomes paler and greener, till it slowly enrichens; the green first becomes broken, then warmer, redder, and finally assumes its original beautiful blood red.

This method of heating is sometimes used as a test, but it is a test which often means the complete ruin of a stone which is not genuine. Another characteristic which, in the eyes of the expert, invariably isolates a real from an artificial ruby is its curious mild brilliance, which as yet has not been reproduced by any scientific method in paste or any other material, but perhaps the safest test of all is the crystalline structure, which identical structure appears in no other stone, though it is possible, by heating alumina coloured with oxide of iron and perhaps also a trace of oxide of chromium to a very high temperature for a considerable time, and then cooling very slowly, to obtain a ruby which is nearly the same in its structure as the real gem; its specific gravity and hardness may perhaps be to standard, and when properly cut, its brilliance would deceive all but an expert.

And as in some real rubies there are found slight hollows corresponding or analogous to the bubbles found in melted glass, it becomes a matter of great difficulty to distinguish the real from the imitation by such tests as hardness, specific gravity, dichroism, and the like, so that in such a case, short of risking the ruin of the stone, ordinary persons are unable to apply any convincing tests.

Therefore, only the expert can decide, by his appreciation of the delicate shade of difference in the light of a true ruby and that of an excellent imitation, and by the distribution of the colour, which -however experienced the chemist may be, or with what care the colouring matter may have been incorporated in the mass -has been found impossible of distribution throughout the body of an artificial stone so perfectly and in the same manner and direction as nature herself distributes it in the genuine.

This alone, even in the closest imitations, is clear to the eye of the expert, though not to the untrained eye, unless the stone is palpably spurious. To one who is accustomed to the examination of precious stones, however perfect the imitation, it is but necessary to place it beside or amongst one or more real ones for the false to be almost instantly identified, and that with certainty.