Reason for Color in Minerals. In Lesson III. we saw that numerous pearl materials cause light that
enters them to separation and take two ways inside the material. Presently all straightforward materials
retain light pretty much; that is, they stop some portion of it, maybe changing over it into heat, and less
light arises than entered the stone. On the off chance that light of all the rainbow tones (red, orange, yellow,
green, blue, violet) is similarly retained, so that there is a similar relative measure 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's anything but a hued stone. Assuming, be that as it may, just blue light prevails with regards to overcoming, the rest
of the[16] white light that entered being ingested inside, we say that we have a blue stone.
Also, the shade of any straightforward material relies on its general level of assimilation of
every one of the tones in white light. That shading which arises most effectively gives its name to
the shade of the stone. In this way a ruby is red since red light prevails with regards to going through the
material far superior to light of some other shading.
Inconsistent Absorption Causes Dichroism. All that has been said so far applies similarly well to
both independently and doubly refracting materials, however in the last sort it is habitually the situation, in those
bearings in which light consistently separates, that the retention isn't equivalent in the two light emissions
light (one is known as the customary beam and the other the phenomenal beam).
For instance, on account of a precious stone of ruby, if white light begins to cross the gem, it not just
isolates into a customary beam and an uncommon beam, however the retention is diverse in[17] the
two cases, and the two beams arise of various shades of red. With most rubies one beam arises
purplish red, the other yellowish red.
It will on the double be seen that if the natural eye could recognize the two beams, we would
have here a stunning strategy for deciding numerous valuable stones. Sadly, the eye does
not break down light, but instead mixes the impact so the independent eye gives yet a helpless methods for
telling whether a stone displays twin tones, or dichroism, as it is called. (The term
means two tones.) An all around prepared eye can, in any case, by review a stone in a few extraordinary
positions, note the distinction in shade of shading brought about by the differential assimilation.
The Dichroscope. Presently, because of the logical laborers, there has been formulated a generally
straightforward and relatively modest instrument called the dichroscope, which empowers one to
tell nearly initially whether a stone is[18] or isn't dichroic. The development is shown in
the going with drawing and portrayal.
THE DICHROSCOPE.
In the event that the onlooker glances through the focal point (A) toward a splendid light, as, for instance, the sky, he
obviously sees two square openings, Fig. 4.
FIG. 3.
A, basic focal point; B, piece of Iceland fight with glass crystals on finishes to settle them; C, square opening.
FIG. 4.
[19]What has happened is that the light going through the square opening (C of Fig. 3) has partitioned
in going through the emphatically doubly refracting Iceland fight (B of Fig. 3) and two pictures of
the square opening are consequently delivered.
On the off chance that now a stone that shows dichroism is held before the square opening and saw toward the
light, two pictures of the stone are seen, one because of its normal beam (which, as was said previously,
will have one tone), and the other because of its exceptional beam (which will have an alternate tone
or then again shade of shading), in this way the shade of the two squares will be unique.
With a separately refracting mineral, or with glass, or with a doubly refracting mineral when seen
in specific ways of the precious stone (which don't yield twofold refraction) the shadings will be similar
in the two squares. In this manner to decide if a red stone is or is anything but a ruby (it very well may be a garnet
or on the other hand glass or a doublet, which are all independently refracting and subsequently can show no dichroism), hold
the stone before the[20] opening in the dichroscope and note whether it produces twin tones.
In the event that there is by all accounts no distinction of shade turn the stone about, as it might have inadvertently been
set so it was seen along its bearing of single refraction. On the off chance that there is still no dichroism
it's anything but a ruby. (Note.— Scientific rubies show dichroism just as common ones, so this test
won't recognize them.)
A dichroscope might be had for from seven to ten dollars, as indicated by the make, and everybody
who bargains in hued stones ought to claim and utilize one.
Not all stones that are doubly refracting show dichroism. White stones obviously can't show
it despite the fact that doubly refracting, and some shaded stones, however unequivocally doubly refracting, do
not show any observable dichroism. The zircon, for instance, is emphatically doubly refracting, yet
shows scarcely any dichroism.
The test is generally valuable for emerald, ruby,[21] sapphire, tourmaline, kunzite and alexandrite, all
of which show checked dichroism.
It is of little use to give here the twin tones for each situation as the shades contrast with various
examples, as per their profundity and kind of shading. The more profound colored stones of any species
show the impact more uniquely than the lighter ones.
The technique is quick and simple—it tends to be applied to mounted stones just as to free ones, and
it can't harm a stone. The understudy ought to, if conceivable, acquire the utilization of a dichroscope and
practice with it on a wide range of stones. He ought to particularly get master in recognizing
between rubies, sapphires, and emeralds, and their impersonations. The lone impersonation (logical
rubies and sapphires are not here classed as impersonations), which is at all liable to bamboozle one who
realizes how to utilize the dichroscope is the emerald trio, made with genuine (yet pale) beryl above
also, underneath, with a meager piece of green glass between. As beryl is[22] doubly refracting to a little
degree, and dichroic, one may maybe be misled by a particularly impersonation if not cautious.
Notwithstanding, the measure of dichroism would be less in such a case than in a genuine emerald of as profound
a shading.
