The Chemistry, Properties and Tests of Precious Stones doc

The Project Gutenberg EBook of TheChemistry, Properties and Tests of Precious Stones, by John Mastin This eBook is for the use of anyone anywhere at no cost and with almost no restrictio

The Project Gutenberg EBook of The

Chemistry, Properties and Tests of

Precious Stones, by John Mastin

This eBook is for the use of anyone

anywhere at no cost and with

almost no restrictions whatsoever You may copy it, give it away or

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Title: The Chemistry, Properties and Tests

of Precious Stones

Author: John Mastin

Release Date: November 26, 2007 [EBook

#23626]

Language: English

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THE CHEMISTRY,

PROPERTIES AND TESTS OF

PRECIOUS STONES

(2nd edition.) 3s 6d.

THIS WORKADAY WORLD (In the

Press.)Henry J Drane

PEPPER'S BOY'S PLAYBOOK OF

THE CHEMISTRY, PROPERTIES

Author of "Parasites of Insects," "The True Analysis of Milk," "Plate-Culture and Staining of Amœbæ," etc., etc.

CHAPTER

I Introductory

II The Origin of Precious Stones

IIIPhysical Properties—(A) CrystallineStructure

IV " " (B) Cleavage

V " " (C) Light

VI " " (D) ColourVII " " (E) HardnessVIII " " (F) Specific

Gravity

IX " " (G) Heat

X " " (H) Magnetic

and Electric Influences

XI The Cutting of Precious Stones

XIIImitations, and Some of the Tests ofPrecious Stones

XIII Various Precious Stones

XIV " " " (continued)

XV " " " "

Some little time ago certain Londondiamond merchants and wholesale dealers

in precious stones made the suggestion to

me to write a work on this section ofmineralogy, as there did not appear to beany giving exactly the information mostneeded

Finding there was a call for such a book Ihave written the present volume in order

to meet this want, and I trust that thishandbook will prove useful, not only tothe expert and to those requiring certaintechnical information, but also to thegeneral public, whose interest in this

entrancing subject may be simply that ofpleasure in the purchase, possession, orcollection of precious stones, or even inthe mere examination of them through theplate-glass of a jeweller's window.

JOHN MASTIN.Totley Brook,

near Sheffield

June 1911.

THE CHEMISTRY, PROPERTIES AND TESTS OF PRECIOUS

STONES

CHAPTER I.

INTRODUCTORY.

What constitutes a precious stone is thequestion which, at the onset, rises in themind, and this question, simple as itseems, is one by no means easy to answer,since what may be considered precious atone time, may cease to be so at another.There are, however, certain mineralswhich possess distinctive features in theirqualities of hardness, colour,transparency, refractability or doublerefractability to light-beams, whichqualities place them in an entirely

different class to the minerals of ametallic nature These particular and non-metallic minerals, therefore, because oftheir comparative rarity, rise pre-eminently above other minerals, andbecome actually "precious."

This is, at the same time, but acomparative term, for it will readily beunderstood that in the case of a suddenflooding of the market with one class ofstone, even if it should be one hithertorare and precious, there would be anequally sudden drop in the intrinsic value

of the jewel to such an extent as perhaps

to wipe it out of the category of preciousstones For instance, rubies werediscovered long before diamonds; thenwhen diamonds were found these were

considered much more valuable till theirabundance made them common, and theybecame of little account Rubies againasserted their position as chief of allprecious stones in value, and in manybiblical references rubies are quoted asbeing the symbol of the very acme ofwealth, such as in Proverbs, chapter iii.,verses 13 and 15, where there are thepassages, "happy is the man that findethwisdom she is more precious thanrubies"—and this, notwithstanding theenormous quantity of them at that timeobtained from the ruby mines of Ophir andNubia, which were then the chief sources

of wealth

It will also be remembered that Josephusrelates how, at the fall of Jerusalem, the

spoil of gold was so great that Syria wasinundated with it, and the value of goldthere quickly dropped to one-half; otherhistorians, also, speaking of this time,record such a glut of gold, silver, andjewels in Syria, as made them of littlevalue, which state continued for someconsiderable period, till the untold wealthbecame ruthlessly and wastefullyscattered, when the normal values slowlyreasserted themselves.

Amongst so many varieties of theseprecious minerals, it cannot be otherwisethan that there should be importantdifferences in their variouscharacteristics, though for a stone to havethe slightest claim to be classed as

"precious" it must conform to several at

least of the following requirements:—Itmust withstand the action of light withoutdeterioration of its beauty, lustre, orsubstance, and it must be of sufficienthardness to retain its form, purity andlustre under the actions of warmth,reasonable wear, and the dust which fallsupon it during use; it must not be subject tochemical change, decomposition,disintegration, or other alteration of itssubstance under exposure to atmosphericair; otherwise it is useless for all practicalpurposes of adornment or ornamentation.There are certain other characteristics ofthese curious minerals which may beclassified briefly, thus:—Some stonesowe their beauty to a wonderful play ofcolour or fire, due to the action of light,

quite apart from the colour of the stoneitself, and of this series the opal may betaken as a type In others, this splendidplay of colour is altogether absent, thecolour being associated with the stoneitself, in its substance, the charm lyingentirely in the superb transparency, theruby being taken as an example of thisclass of stone Others, again, have not onlycolour, but transparency and lustre, as inthe coloured diamonds, whilst thecommoner well-known diamonds areextremely rich in transparency and lustre,the play of light alone showing aconsiderable amount of brilliancy andbeauty of colour, though the stone itself isclear Still others are opaque, or semi-opaque, or practically free from play oflight and from lustre, owing their value

and beauty entirely to their richness ofcolour.

In all cases the value of the stone cannot

be appreciated fully till the gem isseparated from its matrix and polished,and in some cases, such as in that of thediamond, cut in variously shaped facets,

on and amongst which the light rays havepower to play; other stones, such as theopal, turquoise and the like, are cut orground in flat, dome-shaped, or otherform, and then merely polished Itfrequently happens that only a smallportion of even a large stone is of supremevalue or purity, the cutter often retaining

as his perquisite the smaller pieces andwaste These, if too small for setting, areground into powder and used to cut and

polish other stones.

Broadly speaking, the greatest claimwhich a stone can possess in order to beclassed as precious is its rarity To thismay be added public opinion, which is ledfor better or worse by the fashion of themoment For if the comparatively commonamethyst should chance to be madeextraordinarily conspicuous by somesociety leader, it would at once step fromits humbler position as semi-precious, andrise to the nobler classification of a trulyprecious stone, by reason of the demandcreated for it, which would, in allprobability, absorb the available stock torarity; and this despite the more entrancingbeauty of the now rarer stones

The study of this section of mineralogy is

one of intense interest, and byunderstanding the nature, environment,chemical composition and the properties

of the stones, possibility of fraud isaltogether precluded, and there is induced

in the mind—even of those with whom thestudy of precious stones has no partcommercially—an intelligent interest inthe sight or association of what mightotherwise excite no more than a mereglance of admiration or curiosity There isscarcely any form of matter, be it liquid,solid, or gaseous, but has yielded or isnow yielding up its secrets with more orless freedom to the scientist By hismethod of synthesis (which is thescientific name for putting substancestogether in order to form new compoundsout of their union) or of analysis (the

decomposing of bodies so as to divide orseparate them into substances of lesscomplexity), particularly the latter, heslowly and surely breaks down thesubstances undergoing examination intotheir various constituents, reducing thesestill further till no more reduction ispossible, and he arrives at their elements.From their behaviour during the many andvaried processes through which they havepassed he finds out, with unerringaccuracy, the exact proportions of theircomposition, and, in many cases, the cause

of their origin

It may be thought that, knowing all this, it

is strange that man does not himselfmanufacture these rare gems, such as thediamond, but so far he has only succeeded

in making a few of microscopic size,altogether useless except as scientificcuriosities The manner in which theseminute gems and spurious stones aremanufactured, and the methods by whichthey may readily be distinguished fromreal, will be dealt with in due course.The natural stones represent the slowchemical action of water, decay, andassociation with, or near, other chemicalsubstances or elements, combined with theaction of millions of years of time, and theunceasing enormous pressure during thattime of thousands, perhaps millions, oftons of earth, rock, and the like, subjected,for a certain portion at least of that period,

to extremes of heat or cold, all of whichdetermine the nature of the gem So that

only in the earth itself, under strictlynatural conditions, can these raresubstances be found at all in any workablesize; therefore they must be sought afterassiduously, with more or less speculativerisk.

At the very commencement we are faced

with a subject on which mineralogists andgeologists are by no means in fullagreement, and there seems just ground forconsiderable divergence of opinion,according to the line of argument taken It

is a most remarkable fact that, precious asare certain stones, they do not (with a fewexceptions) contain any of the rarermetals, such as platinum, gold, etc., or any

of their compounds, but are composedentirely of the common elements and theirderivatives, especially of those elementscontained in the upper crust of the earth,and this notwithstanding the fact that gemsare often found deep down in the earth.This is very significant, and points to theconclusion that these stones were formed

by the slow percolation of water from thesurface through the deeper parts of the

earth, carrying with it, in solution orsuspension, the chemical constituents ofthe earth's upper crust; time and long-continued pressure, combined with heat orcold, or perhaps both in turn, doing therest, as already mentioned.

The moisture falling in dew and rainbecomes acidulated with carbonic acid,

CO2 (carbon dioxide), from thecombustion and decay of organic matter,vegetation, and other sources, and thismoisture is capable of dissolving certaincalcareous substances, which it takes deepinto the earth, till the time comes when itenters perhaps a division-plane in somerock, or some such cavity, and is unable toget away The hollow becomes filled withwater, which is slowly more and more

charged with the salts brought down, tillsaturated; then super-saturated, so that thesalts become precipitated, or perhapscrystallised out, maybe by the presence ofmore or other salts, or by a change intemperature These crystals then becomepacked hard by further supplies andpressure, till eventually, after the lapse of

ages, a natural gem is found, exactly filling the cavity, and is a precious find in

many cases

If now we try to find its analogy inchemistry, and for a moment consider thecurious behaviour of some well-knownsalts, under different conditions oftemperature, what is taking placeunderground ceases to be mysterious andbecomes readily intelligible

Perhaps the best salt for the purpose, andone easy to obtain for experiment, is thesulphate of sodium—known also asGlauber's Salt.

It is in large, colourless prisms, whichmay soon be dissolved in about three parts

of water, so long as the water does notexceed 60° F., and at this temperature asuper-saturated solution may easily bemade But if the water is heated the saltthen becomes more and more insoluble asthe temperature increases, till it iscompletely insoluble

If a super-saturated solution of thisGlauber's Salt is made in a glass, atordinary atmospheric temperature, andinto this cold solution, without heating, isdropped a small crystal of the same salt,

there will be caused a rise in temperature,and the whole will then crystallise outquite suddenly; the water will beabsorbed, and the whole will solidify into

a mass which exactly fits the inner contour

of the vessel

We have now formed what might be a

precious stone, and no doubt would be, ifcontinuous pressure could be applied to itfor perhaps a few thousand years; at anyrate, the formation of a natural jewel is notgreatly different, and after being subjectedfor a period, extending to ages, to thewashings of moisture, the contact of itscontaining bed (its later matrix), the action

of the changes in the temperature of theearth in its vicinity, it emerges by volcaniceruption, earthquake, landslip and the like,

or is discovered as a rare and valuablespecimen of some simple compound ofearth-crust and water, as simple asGlauber's Salt, or as the pure crystallizedcarbon.

It is also curious to note that in some casesthe stones have not been caused byaqueous deposit in an already existinghollow, but the aqueous infusion has acted

on a portion of the rock on which it rested,absorbing the rock, and, as it were,replacing it by its own substance This isevidenced in cases where gems have beenfound encrusted on their matrix, whichlatter was being slowly transformed to thecharacter of the jewel encrusted, or

"scabbed" on it

The character of the matrix is also in a

great measure the cause of the variety ofthe stone, for it is obvious that the samesalt-charged aqueous solution whichundergoes change in and on ironstonewould result in an entirely differentproduct from that resting on or embedded

in silica

Following out the explanation of theaqueous solution, in which the earth-crustconstituents are secreted, we find that therarer and more precious metals do notgenerally enter into the composition ofprecious stones—which fact mayadvisedly be repeated It is, of course, to

be expected that beryllium will be found

in the emerald, since it is under thespecies beryl, and zirconium in zircon; butsuch instances are the exception, and we

may well wonder at the actions of theinfinite powers of nature, when we reflectthat the rarest, costliest and most beautiful

of all precious stones are the simplest intheir constituents

Thus we find the diamond standing uniqueamongst all gems in being composed ofone element only—carbon—being purecrystallised carbon; a different form fromgraphite, it is true, but, nevertheless, purecarbon and nothing else Therefore, fromits chemical, as well as from itscommercial aspect, the diamond standsalone as the most important of gems

The next in simplicity, whilst being themost costly of all, is the ruby, and withthis may be classed the blue sapphire,seeing that their chemical constituents are

exactly the same, the difference being one

of colour only These have two elements,oxygen and aluminium, which importantconstituents appear also in other stones,but this example is sufficient to provetheir simplicity of origin

Another unique stone is the turquoise, inthat it is the only rare gem essentiallycontaining a great proportion of water,which renders it easily liable todestruction, as we shall see later It is acombination of alumina, water, andphosphoric acid, and is also unique inbeing the only known valuable stonecontaining a phosphate

Turning to the silica series, we again find

a number of gems with two elements only,silica—an important constituent of the

earth's crust—and oxygen—an importantconstituent of atmospheric air In thisgroup may be mentioned the opal,amethyst, agate, rock-crystal, and the like,

as the best known examples, whilstoxygen appears also mostly in the form ofoxides, in chrysoberyl, spinel, and thelike This silica group is extremelyinteresting, for in it, with the exception ofthe tourmaline and a few others, thecomposition of the gems is very simple,and we find in this group such stones asthe chrysolite, several varieties of topaz,the garnet, emerald, etc., etc

Malachite and similar stones are moreornamental than precious, though theycome in the category of precious stones.These are the carbonate series, containing

much carbonic acid, and, as may beexpected, a considerable proportion ofwater in their composition, which watercan, of course, be dispelled by theapplication of heat, but to the destruction

of the stone

From all this will be seen how strong isthe theory of aqueous percolation, for,given time and pressure, water chargedwith earth-crust constituents appears to bethe origin of the formation of all preciousstones; and all the precious stones knownhave, when analysed, been found to bealmost exclusively composed of upper-earth-crust constituents; the othercompounds which certain stones containmay, in all cases, be traced to their matrix,

or to their geological or mineralogical

In contradistinction to this, the essentiallyunderground liquids, with time andpressure, form metallic minerals andmineralise the rocks, instead of forminggems

Thus we see that in a different class ofminerals—compounds of metals with thesulphates, such as sulphuric acid andcompounds; also those containing themetallic sulphides; in cases where themetalliferous ores or the metallic elementsenter into composition with the halogens

—bromine, chlorine, fluorine, and iodine

—in all these, precious stones arecomparatively common, but the stones ofthese groups are invariably those used fordecorative or ornamental purposes, and