The Chemistry, Properties and Tests of Precious Stones Part 2

A well-known simple experiment in physics shows this clearly. A mark on a card or paper is viewed through a piece of double-refracting spar (Iceland spar or clear calcite), when the mark is doubled and two appear. On rotating this rhomb of spar, one of these marks is seen to revolve round the other, which remains stationary, the moving mark pa.s.sing further from the centre in places. When the spar is cut and used in a certain direction, we see but one mark, and such a position is called its optical axis.

_Polarisation_ is when certain crystals possessing double refraction have the power of changing light, giving it the appearance of poles which have different properties, and the polariscope is an instrument in which are placed pieces of double-refracting (Iceland) spar, so that all light pa.s.sing through will be polarised.

Since only crystals possessing the property of double refraction show polarisation, it follows that those of the 1st, or cubic system--in which the diamond stands a prominent example--fail to become polarised, so that when such a stone is placed in the polariscope and rotated, it fails _at every point_ to transmit light, which a double-refracting gem allows to pa.s.s except when its optical axis is placed in the axis of the polariscope, but this will be dealt with more fully when the methods of testing the stones come to be considered.

_Diaphaneity_, or the power of transmitting light:--some rather fine trade distinctions are drawn between the stones in this cla.s.s, technical distinctions made specially for purposes of cla.s.sification, thus:--a "non-diaphanous" stone is one which is quite opaque, no light of any kind pa.s.sing through its substance; a "diaphanous" stone is one which is altogether transparent; "semi-diaphanous" means one not altogether transparent, and sometimes called "sub-transparent." A "translucent"

stone is one in which, though light pa.s.ses through its substance, sight is not possible through it; whilst in a "sub-translucent" stone, light pa.s.ses through it, but only in a small degree.

The second physical property of light is seen in those stones which owe their beauty or value to REFLECTION: this again may be dependent on l.u.s.tre, or Colour.

~l.u.s.tre.~--This is an important characteristic due to reflection, and of which there are six varieties:--([alpha]) adamantine (which some authorities, experts and merchants subdivide as detailed below); ([beta]) pearly; ([gamma]) silky; ([delta]) resinous; ([epsilon]) vitreous; ([zeta]) metallic. These may be described:--

([alpha]) Adamantine, or the peculiar l.u.s.tre of the diamond, so called from the l.u.s.tre of adamantine spar, which is a form of corundum (as is emery) with a diamond-like l.u.s.tre, the hard powder of which is used in polishing diamonds. It is almost pure anhydrous alumina (Al_{2}O_{3}) and is, roughly, four times as heavy as water. The l.u.s.tre of this is the true "adamantine," or diamond, brilliancy, and the other and impure divisions of this particular l.u.s.tre are: _splendent_, when objects are reflected perfectly, but of a lower scale of perfection than the true "adamantine" standard, which is absolutely flawless. When still lower, and the reflection, though maybe fairly good, is somewhat "fuzzy," or is confused or out of focus, it is then merely _shining_; when still less distinct, and no trace of actual reflection is possible (by which is meant that no object can be reproduced in any way to define it, as it could be defined in the reflection from still water or the surface of a mirror, even though imperfectly) the stone is then said to _glint_ or _glisten_. When too low in the scale even to glisten, merely showing a feeble l.u.s.tre now and again as the light is reflected from its surface in points which vary with the angle of light, the stone is then said to be _glimmering_. Below this, the definitions of l.u.s.tre do not go, as such stones are said to be _l.u.s.treless_.

([beta]) Pearly, as its name implies, is the l.u.s.tre of a pearl.

([gamma]) Silky, possessing the sheen of silk, hence its name.

([delta]) Resinous, also explanatory in its name; amber and the like come in this variety.

([epsilon]) Vitreous. This also explains itself, being of the l.u.s.tre of gla.s.s, quartz, etc.; some experts subdividing this for greater defining accuracy into the "sub-vitreous" or lower type, for all but perfect specimens.

([zeta]) Metallic or Sub-metallic. The former when the l.u.s.tre is perfect as in gold; the latter when the stones possess the less true l.u.s.tre of copper.

~Colour.~--Colour is an effect entirely dependent upon light, for in the total absence of light, such as in black darkness, objects are altogether invisible to the normal human eye. In daylight, also, certain objects reflect so few vibrations of light, or none, that they appear grey, black, or jet-black; whilst those which reflect all the rays of which light is composed, and in the same number of vibrations, appear white. Between these two extremes of _none_ and _all_ we find a wonderful play and variety of colour, as some gems allow the red rays only to pa.s.s and therefore appear red; others allow the blue rays only and these appear blue, and so on, through all the shades, combinations and varieties of the colours of which light is composed, as revealed by the prism. But this is so important a matter that it demands a chapter to itself.

The third physical property of light, PHOSPh.o.r.eSCENCE, is the property possessed by certain gems and minerals of becoming phosph.o.r.escent on being rubbed, or on having their temperature raised by this or other means.

It is difficult to say exactly whether this is due to the heat, the friction, or to electricity. Perhaps two or all of these may be the cause, for electricity is developed in some gems--such as the topaz--by heat, and heat by electricity, and phosph.o.r.escence developed by both.

For example, if we rub together some pulverised fluorspar in the dark, or raise its temperature by the direct application of heat, such as from a hot or warm iron, or a heated wire, we at once obtain excellent phosph.o.r.escence. Common quartz, rubbed against a second piece of the same quartz in the dark, becomes highly phosph.o.r.escent. Certain gems, also, when merely exposed to light--sunlight for preference--then taken into a darkened room, will glow for a short time. The diamond is one of the best examples of this kind of phosph.o.r.escence, for if exposed to sunlight for a while, then covered and rapidly taken into black darkness, it will emit a curious phosph.o.r.escent glow for from one to ten seconds; the purer the stone, the longer, clearer and brighter the result.

CHAPTER VI.

PHYSICAL PROPERTIES.

D--COLOUR.

Colour is one of the most wonderful effects in nature. It is an attribute of light and is not a part of the object which appears to be coloured; though all objects, by their chemical or physical composition, determine the number and variety of vibrations pa.s.sed on or returned to the eye, thus fixing their own individual colours.

We have also seen that if an _equal_ light-beam becomes obstructed in its pa.s.sage 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 pa.s.s 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 a.s.sumes 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, bra.s.s 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 pa.s.s 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 colours 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 cla.s.sified 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_ colour. 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 cla.s.sified, 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-gla.s.s, 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 "n.o.ble"

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 cla.s.s 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 "n.o.ble" 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 gla.s.sy or vitreous l.u.s.tre, and is found almost exclusively in the form of globules, or cl.u.s.ters 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 cla.s.s 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 ma.s.s of coloured light slowly changes, usually to its complementary. Thus in this cla.s.s 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 gla.s.s, 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.

CHAPTER VII.

PHYSICAL PROPERTIES.

E--HARDNESS.

Hardness is perhaps one of the most important features in a stone, especially those of the "gem" series, for no matter how colour, l.u.s.tre, general beauty and even rarity may ent.i.tle a stone to the designation "precious," unless it possesses great hardness it cannot be used as a gem or jewel.

Consequently, the hardness of jewels is a matter of no small importance, and by dint of indefatigable research, in tests and comparison, all known precious stones have been cla.s.sified in various scales or degrees of hardness. The most popular and reliable table is that of Mohs, in which he takes talc as the softest of the rarer minerals and cla.s.ses this as No. 1; from that he goes by gradual steps to the diamond, the hardest of the stones, which he calls No. 10, and between these two all other gems are placed. Here is given a complete list of Mohs's arrangement of stones, according to their hardness, beginning at No. 1, thus:--

Talc 1 Rock salt 2 Amber 2-1/2 Calcite 3 Malachite 3-1/2 Jet 3-1/2 Fluorspar 4 Apat.i.te 5 Dioptase 5 Kyanite (various) 5-7 Hauynite 5-1/2 Haemat.i.te 5-1/2 Lapis lazuli 5-1/2 Moldavite (various) 5-1/2-6-1/2 Rhodonite 5-1/2-6-1/2 Obsidian 5-1/2 Sphene 5-1/2 Opal (various) 5-1/2-6-1/2 Nephrite 5-3/4 Chrysolite 6-7 Felspar 6 Adularia 6 Amazon stone 6 Diopside 6 Iron pyrites 6 Labradorite 6 Turquoise 6 Spodumene 6-1/2-7 The Chalcedony group which embraces the Agate, Carnelian, etc. 6-1/2 Demantoid 6-1/2 Epidote 6-1/2 Idocrase 6-1/2 Garnets (see also "Red Garnets" below) 6-1/2-7-1/2 Axinite 6-3/4 Jadeite 6-3/4 Quartz, including Rock-crystal, Amethyst, Jasper, Chrysoprase Citrine, etc. 7 Jade 7 Dichorite (water sapphire) 7-7-1/2 Cordierite 7-1/4 Red Garnets (see also Garnets above) 7-1/4 Tourmaline 7-1/4 Andalusite 7-1/2 Euclase 7-1/2 Staurolite 7-1/2 Zircon 7-1/2 Emerald, Aquamarine, or Beryl 7-3/4 Phenakite 7-3/4 Spinel 8 Topaz 8 Chrysoberyl 8-1/2 The Corundum group embracing the Ruby, Sapphire, etc. 9 Diamond 10

(See also list of stones, arranged in their respective colours, in Chapter XII.)

The method of testing is very simple. A representative selection of the above stones, each with a sharp edge, is kept for the purpose of scratching and being scratched, and those usually set apart for tests in the various groups, are as follows:--

1 Talc 2 Rock-salt, or Gypsum 3 Calcite 4 Fluorspar 5 Apat.i.te 6 Felspar 7 Quartz 8 Topaz 9 Corundum 10 Diamond

The stone under examination may perhaps first be somewhat roughly cla.s.sified by its colour, cleavage, and general shape. One of these standard stones is then gently rubbed across its surface and then others of increasingly higher degrees, till no scratch is evident under a magnifying gla.s.s. Thus if quartz ceases to scratch it, but a topaz will do so, the degree of hardness must lie between 7 and 8. Then we reverse the process: the stone is pa.s.sed over the standard, and if both quartz and topaz are scratched, then the stone is at least equal in hardness to the topaz, and its cla.s.sification becomes an easy matter.

Instead of stones, some experts use variously-tempered needles of different qualities and compositions of iron and steel. For instance, a finely-tempered ordinary steel needle will cut up to No. 6 stones; one made of tool steel, up to 7; one of manganese steel, to 7-1/2; one made of high-speed tool steel, to 8 and 8-1/2, and so on, according to temper; so that from the scratch which can be made with the finger-nail on mica, to the hardness of the diamond, which diamond alone will scratch readily, the stones may be picked out, cla.s.sified and tested, with unerring accuracy.

It will thus be seen how impossible it is, even in this one of many tests, for an expert to be deceived in the purchase of precious stones, except through gross carelessness--a fault seldom, if ever, met with in the trade. For example--a piece of rock-crystal, chemically coloured, and cut to represent a ruby, might appear so like one as to deceive a novice, but the mere application to its surface of a real ruby, which is hardness 9, or a No. 9 needle, would reveal too deep or powdery a scratch; also its possibility of being scratched by a topaz or a No. 8 needle, would alone prove it false, for the corundum group, being harder than No. 8, could not be scratched by it. So would the expert go down the scale, the tiny scratches becoming fainter as he descended, because he would be approaching more nearly the hardness of the stone under test, till he arrived at the felspar, No. 6, which would be too soft to scratch it, yet the stone would scratch the felspar, but not zircon or andalusite, 7-1/2, or topaz, 8, so that his tests would at once cla.s.sify the stone as a piece of cut and coloured quartz, thus confirming what he would, at the first sight, have suspected it to be.