LESSON IV
ABSORPTION AND DICHROISM
Cause of Color in Minerals. In [Lesson III.] we saw that many gem materials cause light that enters them to divide and take two paths within the material. Now all transparent materials absorb light more or less; that is, they stop part of it, perhaps converting it into heat, and less light emerges than entered the stone. If light of all the rainbow colors (red, orange, yellow, green, blue, violet) is equally absorbed, so that there is the same relative amount of each in the light that comes out as in the light that went into a stone, we say that the stone is a white stone; that is, it is not a colored stone. If, however, only blue light succeeds in getting through, the rest of the white light that entered being absorbed within, we say that we have a blue stone.
Similarly, the color of any transparent material depends upon its relative degree of absorption of each of the colors in white light. That color which emerges most successfully gives its name to the color of the stone. Thus a ruby is red because red light succeeds in passing through the material much better than light of any other color.
Unequal Absorption Causes Dichroism. All that has been said so far applies equally well to both singly and doubly refracting materials, but in the latter sort it is frequently the case, in those directions in which light always divides, that the absorption is not equal in the two beams of light (one is called the ordinary ray and the other the extraordinary ray).
For example, in the case of a crystal of ruby, if white light starts to cross the crystal, it not only divides into an ordinary ray and an extraordinary ray, but the absorption is different in the two cases, and the two rays emerge of different shades of red. With most rubies one ray emerges purplish red, the other yellowish red.
It will at once be seen that if the human eye could distinguish between the two rays, we would have here a splendid method of determining many precious stones. Unfortunately, the eye does not analyze light, but rather blends the effect so that the unaided eye gives but a poor means of telling whether or not a stone exhibits twin colors, or dichroism, as it is called. (The term signifies two colors.) A well-trained eye can, however, by viewing a stone in several different positions, note the difference in shade of color caused by the differential absorption.
The Dichroscope. Now, thanks to the scientific workers, there has been devised a relatively simple and comparatively inexpensive instrument called the dichroscope, which enables one to tell almost at a glance whether a stone is or is not dichroic. The construction is indicated in the accompanying drawing and description.
The Dichroscope.
If the observer looks through the lens (A) toward a bright light, as, for example, the sky, he apparently sees two square holes, [Fig. 4].
Fig. 3.
A, simple lens; B, piece of Iceland spar with glass prisms on ends to square them up; C, square hole.
Fig. 4.
What has happened is that the light passing through the square hole (C of [Fig. 3]) has divided in passing through the strongly doubly refracting Iceland spar (B of [Fig. 3]) and two images of the square hole are thus produced.
If now a stone that exhibits dichroism is held in front of the square hole and viewed toward the light, two images of the stone are seen, one due to its ordinary ray (which, as was said above, will have one color), and the other due to its extraordinary ray (which will have a different color or shade of color), thus the color of the two squares will be different.
With a singly refracting mineral, or with glass, or with a doubly refracting mineral when viewed in certain directions of the crystal (which do not yield double refraction) the colors will be alike in the two squares. Thus to determine whether a red stone is or is not a ruby (it might be a garnet or glass or a doublet, all of which are singly refracting and hence can show no dichroism), hold the stone before the hole in the dichroscope and note whether or not it produces twin colors. If there seems to be no difference of shade turn the stone about, as it may have accidentally been placed so that it was viewed along its direction of single refraction. If there is still no dichroism it is not a ruby. (Note.—Scientific rubies exhibit dichroism as well as natural ones, so this test will not distinguish them.)
A dichroscope may be had for from seven to ten dollars, according to the make, and everyone who deals in colored stones should own and use one.
Not all stones that are doubly refracting exhibit dichroism. White stones of course cannot exhibit it even though doubly refracting, and some colored stones, though strongly doubly refracting, do not exhibit any noticeable dichroism. The zircon, for example, is strongly doubly refracting, but shows hardly any dichroism.
The test is most useful for emerald, ruby, sapphire, tourmaline, kunzite and alexandrite, all of which show marked dichroism.
It is of little use to give here the twin colors in each case as the shades differ with different specimens, according to their depth and type of color. The deeper tinted stones of any species show the effect more markedly than the lighter ones.
The method is rapid and easy—it can be applied to mounted stones as well as to loose ones, and it cannot injure a stone. The student should, if possible, obtain the use of a dichroscope and practice with it on all sorts of stones. He should especially become expert in distinguishing between rubies, sapphires, and emeralds, and their imitations. The only imitation (scientific rubies and sapphires are not here classed as imitations), which is at all likely to deceive one who knows how to use the dichroscope is the emerald triplet, made with real (but pale) beryl above and below, with a thin strip of green glass between. As beryl is doubly refracting to a small degree, and dichroic, one might perhaps be deceived by such an imitation if not careful. However, the amount of dichroism would be less in such a case than in a true emerald of as deep a color.
Those who wish to study further the subject of dichroism should see Gem-Stones, by G. F. Herbert-Smith, Chapter VII., pp. 53-59, or see A Handbook of Precious Stones, by M. D. Rothschild, Putnam's, pp. 14-16.
LESSON V
SPECIFIC GRAVITY
The properties so far considered as serving to distinguish precious stones have all depended upon the behavior of the material toward light.
These properties were considered first because they afford, to those acquainted with their use, very rapid and sure means of classifying precious stones.
Density of Minerals. We will next consider an equally certain test, which, however, requires rather more time, apparatus, and skill to apply.
Each kind of precious stone has its own density. That is, if pieces of different stones were taken all of the same size, the weights would differ, but like-sized pieces of one and the same material always have the same weight. It is the custom among scientists to compare the densities of substances with the density of water. The number which expresses the relation between the density of any substance and the density of water is called the specific gravity number of the substance. For example, if, size for size, a material, say diamond, is 3.51 times as heavy as water, its specific gravity is 3.51. It will be seen that since each substance always has, when pure, the same specific gravity, we have here a means of distinguishing precious stones. It is very seldom, if ever, the case that we find any two precious stones of the same specific gravity. A few stones have nearly the same specific gravities, and in such cases it is well to apply other tests also. In fact one should always make sure of a stone by seeing that two or three different tests point to the same species.
We must next find out how to determine the specific gravity of a precious stone. If the shape of a stone were such that the volume could be readily calculated, then one could easily compare the weight with the volume or with the weight of the same volume of water, and thus get the specific gravity (for a specific gravity number really tells how much heavier a piece of material is than the same volume of water).
Unfortunately the form of most precious stones is such that it would be very difficult to calculate the volume from the measurements, and the latter would be hard to make accurately with small stones. To avoid these difficulties the following ingenious method has been devised:
If a stone is dropped into water it pushes aside, or displaces, a body of water exactly equal in volume to itself. If the water thus displaced were caught and weighed, and the weight of the stone then divided by the weight of the water displaced, we would have the specific gravity number of the stone.
This is precisely what is done in getting the specific gravity of small stones. To make sure of getting an accurate result for the weight of water displaced the following apparatus is used.
Fig. 5.
A, Flask-like Bottle; B, Indicates Ground Glass Stopper;
C, Shows Hole Drilled through Stopper.
The Specific Gravity Bottle. A small flask-like bottle (see [Fig. 5]) is obtained. This has a tightly fitting ground glass stopper (B). The stopper has a small hole (C) drilled through it lengthwise. If the bottle is filled with water, and the stopper dropped in and tightened, water will squirt out through the small hole in the stopper. On wiping off stopper and bottle we have the bottle exactly full of water. If now the stopper is removed, the stone to be tested (which must of course be smaller than the neck of the bottle) dropped in, and the stopper replaced, exactly as much water will squirt out as is equal in volume to the stone that was dropped in.
If we had weighed the full bottle with the stone on the pan beside it, and then weighed the bottle with the stone inside it we could now, by subtracting the last weight from the first, find out how much the water, that was displaced, weighed. This is precisely the thing to do. The weight of the stone being known we now have merely to divide the weight of the stone by the weight of the displaced water, and we have the specific gravity number. Reference to a table of specific gravities of precious stones will enable us to name our stone. Such a table follows this lesson.
A Sample Calculation. The actual performance of the operation, if one is skilled in weighing, takes less time than it would to read this description. At first one will be slow, and perhaps one should read and re-read this lesson, making sure that all the ideas are clear before trying to put them in practice.
A sample calculation may help make the matter clearer, so one is appended:
| Weight of bottle + stone (outside) | = | 53.51 | carats |
| Weight of bottle + stone (inside) | = | 52.51 | carats |
| Weight of water displaced | = | 1.00 | carat |
| Weight of stone | = | 3.51 | carats |
| Specific gravity = | Weight of stone | = | 3.51 | = 3.51 Sp. g. |
| Weight of water | 1.00 |
In this case the specific gravity being 3.51, the stone is probably diamond (see table), but might be precious topaz, which has nearly the same specific gravity.
It is assumed that the jeweler will weigh in carats, and that his balance is sensitive to .01 carat. With such a balance, and a specific gravity bottle (which any scientific supply house will furnish for less than $1) results sufficiently accurate for the determination of precious stones may be had if one is careful to exclude air bubbles from the bottle, and to wipe the outside of the bottle perfectly dry before each weighing. The bottle should never be held in the warm hands, or it will act like a thermometer and expand the water up the narrow tube in the stopper, thus leading to error. A handkerchief may be used to grasp the bottle.
Table of Specific Gravities of the Principal Gem Materials
| Beryl (Emerald) | 2.74 | |
| Chrysoberyl (Alexandrite) | 3.73 | |
| Corundum (Ruby, sapphire, "Oriental topaz") | 4.03 | |
| Diamond | 3.52 | |
| Garnet | (Pyrope) | 3.78 |
| " | (Hessonite) | 3.61 |
| " | (Demantoid, known in the trade as "Olivine") | 3.84 |
| " | (Almandite) | 4.05 |
| Opal | 2.15 | |
| Peridot | 3.40 | |
| Quartz (Amethyst, common topaz) | 2.66 | |
| Spinel (Rubicelle, Balas ruby) | 3.60 | |
| Spodumene (Kunzite) | 3.18 | |
| Topaz (precious) | 3.53 | |
| Tourmaline | 3.10 | |
| Turquoise | 2.82 | |
| Zircon, | lighter variety | 4.20 |
| " | heavier variety | 4.69 |
For a more complete and scientific discussion of specific gravity determination see Gem-Stones, by G. F. Herbert-Smith, Chapter VIII., pp. 63-77; or see, A Handbook of Precious Stones, by M. D. Rothschild, pp. 21-27, for an excellent account with illustrations; or see any physics text-book.
LESSON VI
SPECIFIC GRAVITY DETERMINATIONS
Weighing a Gem in Water. In the previous lesson it was seen that the identity of a precious stone may be found by determining its specific gravity, which is a number that tells how much heavier the material is than a like volume of water. It was not explained, however, how one would proceed to get the specific gravity of a stone too large to go in the neck of a specific gravity bottle. In the latter case we resort to another method of finding how much a like volume of water weighs. If the stone, instead of being dropped into a perfectly full bottle of water (which then overflows), be dropped into a partly filled glass or small beaker of water, just as much water will be displaced as though the vessel were full, and it will be displaced upward as before, for lack of any other place to go. Consequently its weight will tend to buoy up or float the stone by trying to get back under it, and the stone when in water will weigh less than when in air. Anyone who has ever pulled up a small anchor when out fishing from a boat will recognize at once that this is the case, and that as the anchor emerges from the water it seems to suddenly grow heavier. Not only does the stone weigh less when in the water, but it weighs exactly as much less as the weight of the water that was displaced by the stone (which has a volume equal to the volume of the stone). If we weigh a stone first in the air, as usual, and then in water (where it weighs less), and then subtract the weight in water from the weight in air we will have the loss of weight in water, and this equals the weight of an equal volume of water, which is precisely what we got by our bottle method.
We now need only divide the weight in air by the loss of weight in water, and we shall have the specific gravity of the stone.
Fig. 6.
To actually weigh the stone in water we must use a fine wire to support the stone. We must first find how much this wire itself weighs (when attached by a small loop to the hook that supports the balance pan and trailing partly in the water, as will be the case when weighing the stone in water). This weight of the wire must of course be deducted to get the true weight of the stone in water. The beaker of water is best supported by a small table that stands over the balance pan. One can easily be made out of the pieces of a cigar box. (See [Fig. 6].)
The wire that is to support the stone should have a spiral at the bottom in which to lay the gem, and this should be so placed that the latter will be completely submerged at all times, but not touching bottom or sides of the beaker.
Example of data, and calculation, when getting specific gravity by the method of weighing in water:
| Weight of stone | = | 4.02 | carats |
| Weight of stone (plus wire) in water | = | 3.32 | carats |
| Weight of wire | = | .30 | carat |
| True weight of stone in water | = | 3.02 | carats |
| Loss of weight in water | = | 1.00 | carat |
| Specific gravity = | Weight of stone | = | 4.02 | = 4.02 |
| Loss in water | 1.00 |
Here the specific gravity, 4.02 would indicate some corundum gem (ruby or sapphire), and the other characters would indicate at once which it was.
The student who means to master the use of the two methods given in [Lessons V.] and [VI.] should proceed to practice them with stones of known specific gravities until he can at least get the correct result to the first decimal place. It is not to be expected that accurate results can be had in the second decimal place, with the balances usually available to jewelers. When the learner can determine specific gravities with some certainty he should then try unknown gems.
The specific gravity method is of especial value in distinguishing between the various colorless stones, as, for example, quartz crystal, true white topaz, white sapphire, white or colorless beryl, etc. These are all doubly refractive, have no color, and hence no dichroism, and unless one has a refractometer to get the refractive index, they are difficult to distinguish. The specific gravities are very different, however, and readily serve to distinguish them. It should be added that the synthetic stones show the same specific gravities as their natural counterparts, so that this test does not serve to detect them.
Where many gems are to be handled and separated by specific gravity determinations, perhaps the best way to do so is to have several liquids of known specific gravity and to see what stones will float and what ones will sink in the liquids. Methylene iodide is a heavy liquid (sp. g. 3.32), on which a "quartz-topaz," for example, sp. g. 2.66, would float, but a true topaz, sp. g. 3.53, would sink in it. By diluting methylene iodide with benzol (sp. g. 0.88) any specific gravity that is desired may be had (between the two limits 0.88 and 3.32). Specimens of known specific gravity are used with such liquids and their behavior (as to whether they sink or float, or remain suspended in the liquid,) indicates the specific gravity of the liquid. An unknown stone may then be used and its behavior noted and compared with that of a known specimen, whereby one can easily find out whether the unknown is heavier or lighter than the known sample.
An excellent account of the detail of this method is given in G. F. Herbert-Smith's Gem-Stones, pages 64-71, of Chapter VIII., and various liquids are there recommended. It is doubtful if the practical gem dealer would find these methods necessary in most cases. Where large numbers of many different unknown gems have to be determined it would pay to prepare, and standardize, and use such solutions.