546 



NA TURE 



[April 4, 1895 



sidered complete which did not include the bacteriological 

 evidence ; that is to say, the isolation and microscopic examina- 

 tion in each case of the specific bacillus. %Yhat is more, such 

 evidence is proof positive of the existence of the disease. 



Now the only characters at all generally employed by persons 

 connected with the trade in precious stones are two — namely, 

 the hardness and the specific gravity or weightiness. 



I f a stone scratches quartz, and is scratched by topaz, it is said 

 to have a hardness between that of quartz and that of topaz ; 

 if it scratches topaz, but is scratched by sapphire, it is said to 

 have a hardness between that of topaz and that of sapphire. 

 All minerals, including the gem-stone?, have leen tabulated 

 according to their hardness with reference to ten standard stones, 

 of which the diamond, the haidest of all known substances, 

 heads the list. If, for exDmple, a red stone, supposed to be a 

 ruby, is found to be only about as hard as topaz, it cannot be a 

 true ruby, but must be a spinel luby, which is quite a dift'erent 

 thing ; or if it is sufficiently soft to be scratched by rock-crystal, 

 it is probably a red garnet. 



This test is obviously a very rude one in more senses than 

 one. Not only does everything depend upon the nature of the 

 scratching pari, whether it is a sharp corner or a curved surface, 

 and upon the direction in which the scratch is made ; but, to 

 say the least, the surface of a gem is certainly not improved by 

 scratching. 



The second lest — that of the weightiness — is a really accurate 

 and scientific one, provided that it be made by means of a 

 delicate chemical balance. .\ stone which is, bulk for bulk, 

 three times as heavy as water, is said to have a specific gravity 

 of 3 ; one such as topaz, which is three and a half times as 

 heavy as water, is said to have a specific gravity of 3'5. The 

 ordinary method is to weigh the stone, suspended by a thread, 

 first in air, and then immersed in water. The dilTerence is 

 exactly the weight of the water displaced by the stone, and so 

 the specific gravity is easily found. 



The objections to this method are, firstly, that it is too 

 laborious ; and secondly, that it is not applicable when the stone 

 is very small, because it is then impossible to weigh it with 

 accuracy under water. I should not rely upon the specific 

 gravity of a stone under two carats in weight as determined by 

 this method. A method which I shall presently describe is 

 perfectly free from both these ol>jections. 



Incredible as it may seem, the estimation of hardness and 

 the specific gravity are the only attempts at anything like 

 scientific measurement ever made in the ordinary course of 

 business applied to stones ; and even then the weightiness is 

 usually estimated merely by poising the stone in the hand. For 

 the test they are identified by their colour, their fire or sparkle, 

 their lustre and their general appearance. 



In a lecture delivered to the .Society of Arts in 1881, Prof. 

 Church drew attention to the necessity of scientific methods (or 

 this purpose, and has more than once, on subsequent occasions, 

 reiterated his plea. I propose to dwell more particularly on 

 improvements which have been introduced since the date of his 

 lecture, and to indicate how one may, by simple practical tests, 

 which require little special knowledge, distinguish with cer- 

 tainly ail gem-stones without in any way injuring them. 



Chemical analysis is, by the very nature of the problem, out 

 of the question, for in order to make an analysis, or to apply 

 the simplest chemical test, it is necessary to destroy a part of 

 the material ; and this cannot be done, at any rate in the case of 

 a cut stone. 



We can begin by dismissing the hardness as a characler 

 which it is really unnecessary to determine, except to identify 

 diamond or to distinguish a real stone from paste : here, 1 

 know, I shall earn a rebuke from the orthodox mineralogist 

 who, in order lo pursue Ihe study of what should be a peaceful 

 science, arms himself with a knife, and proceeds lo scratch 

 everything which he comes acros-. 



The weapons which I would recommend are of a milder 

 nature : the microscope, Ihe spectroscope, the goniometer, and 

 the dichroscope. 



.Among Ihe available characters of gems, first and foremost 

 arc the optical properties ; that is lo say, the appearances seen 

 when we look at them, or through them, in various ways. 



The extent 10 which a ray of light is refr.icled on entering and 

 leaving a transparent stone, is a characlerislic prn|ierty most use- 

 ful for delcrminatiun. As everyone knows, a stick half immersed 

 in water appears bent owing lo Ihe relr.action of light on passing 



NO. 1327, VOL. 51] 



out of the water ; if it is immersed in a more highly refractive 

 liquid, it appears more bent. 



To ascertain the refractive power of any transparent sub- 

 stance like glass, a prism-shaped piece is cut from it, and the 

 extent to which a ray of li^ht is refracted on passing through 

 the prism is measured by the goniometer, an instrument found 

 in ever)' physical laboratory. 



I have not seen this recommended as a method to be 

 practically used, because it is commonly supposed that a special 

 prism must be cut from the stone for the purpose. For the 

 benefit of those who possess a goniometer, I may say that it is a 

 method which I constantly apply, and find most useful for 

 unmounted cut stones. It is always possible to find two of the 

 facets which form a convenient angle, and, after inking over the 

 remainder of the stone, to trace the ray passing through these 

 two facets, and so to measure with absolute accuracy not only 

 the refraction but the double refraction of the stone ; moreover, 

 this method is applicable to any stoue, however great its refrac- 

 tive power. 



Another simple plan which can be used by any one, but un- 

 fortunately only for stones of comparatively low refractive power, 

 has been invented during the last few years. This delightfully 

 simple litile instrument, known as the reflectomeler, consists of 

 a hemispherical glass lens viewed by an eye-piece containing a 

 graduated scale ; it need only be pressed against the plane 

 surface of a cut stone previously touched with a drop of liquid 

 of higher refractive power than the stone itself. On looking 

 into the eyepiece a shade is seen over half the field of view, 

 and its edge crosses the scale at a point which gives the exact 

 refractive index of the stone. The best available liquid is 

 nionobromo naphthalene, which has a refractive power higher 

 than that of topaz, and enables one at a glance to distinguish a 

 cu* topaz or any less brillinnt gem-stone. 



Most useful, again, are the so-called interference figures — 

 the appearances seen on looking through a transparent stone by 

 means of a polarising microscope, such as is used by every 

 jjeologist. There is, of course, nothing new in these figures; 

 ihey are now employed by geologists in the study of rocks, and 

 even sometimes by those whose business it is to distinguish 

 precious stones. 



Without endeavouring to explain the nature of these figures, 

 except to say that they are due to the double refraction of the 

 crystal, it is easy to show that by looking at a stone through a 

 microscope, one may see something very characteristic. 



(The interference figures of several minerals were thrown 

 upon the screen by means of a projection app.iraius lent by 

 Prof. Ayrton ; sapphire, tourmaline, and emerald were shown 

 to give coloured circular rings intersected by a bhack cross ; 

 sphcne and chrysoberyl, coloured oval rings intersected by a 

 hyperbola ; and iiu.irtz, coloured circular rings with a black 

 cross having a tinted centre.) 



This beautiful method is not employed neiirly so largely as it 

 deserves, because most people find it difficult. In order to see 

 the figure it is necessary lo look through any given crystal in 

 one certain direction. (The stones ust'd for projection were 

 plates approprialely cut for the purpose). Now il m.ay happen 

 that a facetted stone is so cut that to look along the required 

 direction would be to look through an angular corner ; and 

 every one knows that it is not possible to look through a 

 pointed corner, owing to the refraction of the light. For this 

 reason when an unmounted cut jewel is held under the polaris- 

 ing microscope, and yieltls no interference figure when turned 

 about into various positions, it is usually given up .as hopeless. 

 But obviously we have only to immerse the stone in some 

 liquid having nearly the same refractive ))ower as itself, in 

 order to eliminate the dilhculty due to refr.aclion. I find that 

 if the stone be placed in a small tube filled with oil or glycerine, 

 and held in various positions, the interference figure can always 

 be seen. Little more than a year ago, a small facetted stone 

 of peculiar appearance was sent to me, which had deceived the 

 experts to whom it had been shown, although agreeing in some 

 respects with quartz, and was supposed to be a new stone. But 

 by immersing it in oil in a hollow glass sphere, I was able to 

 see the characteristic interference figure of quartz. When a 

 stone has Ihe refraction, Ihe double refraction, the specific 

 gravity, and the characteristic interference figure of quartz — it 

 is quartz and nothing else. 



Other optical characters of great value are those resulting 

 from the absorption of the light in its passage through a crystal; 



