Gemology
Author: Alain Darbellay GGGems
Article:
© 2004 gggems.com All rights reserved.
Gemology is by definition the study or the science of the precious stones. One
knows approximately 4000 minerals produced by the terrestrial body, among which
less than 100 produce specimens considered as precious stones.
So that a mineral is a gem, it must have three fundamental virtues. Beauty,
durability and scarcity.
Beauty; it depends on the following characters:
- Transparent minerals: The glare
(brightness) dispersion (fire) the transparency (purity) intensity of the color
(for the colored stones)
- Translucent minerals : Color or the reflection,
opalescence, the changing luster or asterism (in the case of the phenomenal
stones)
- Opaque minerals: Purity and intensity of the color, the propensity with
reflecting their color well and taking the polish. All these factors of beauty
are highlighted by the size and polishing.
Durability depends on resistance to the mechanical or chemical attacks to which
the stones are subjected. Dust often containing quartz particles. It is
preferable that a gem is as hard as this mineral.
Rarity: A gem often comes from a common and very widespread mineral species, but
it consists of a specimen in which exist seldom joined together qualities.
Gemology is the study of the significant properties of the gems
it serves of the apparatuses or instruments necessary to the
observation and the measurement of these properties.
The gemologist must be able to differentiate the natural stones from the
synthetic stones using his instruments. - Here are the principal instruments
used by the gemologist:
The
preliminary examination at the eye then with the loupe "10x" is thus of primary
importance because the characteristic signs of a gem will help the gemologist to
choose effective instruments for the identification.
The characteristic signs of a gem which one can observe at the
naked eye or with the loupe are as follows:
- Texture: rough gem or opaque
- Gloss: diamantine, vitreous or resinous etc.
- Substantial dispersion (indication of a high index of
refraction) or weak, presence or absence of cleavages.
- Characteristic fracture: vitreous, granulous or splintering etc.
- Assembled stone: doublet or triplet.
- Corners between the facets: sharp or round, often with the
concave facets (indication of glass or a cast plastic) or
notched (stone of low hardness).
Abundant inclusions: which make the gems translucent, and by
consequence, the optical signs could not be obtained under the
polariscope. Moreover, one could not insist enough on the
importance of a perfect cleanliness of the stones to examine.
The cleaning of the unset stones is easy; it is enough to wipe
them with a clean handkerchief. A good cleaning can be obtained
with water accompanied by a little liquid detergent. To avoid
dirtying the stones, the specialized tweezers represent a
pleasant complement of the fingers. - Scale determining the
purity of a stone.
- FL Flaw less
- NFL Near flaw less
- LI1 Lightly included 1
- LI2 Lightly included 2
- MI1 Medium included 1
- MI2 Medium included 2
- MI3 Medium included 3
- VI1 Very included 1
- VI2 Very included 2
- HI Highly included
Note that our camera can grasp very
small inclusions almost invisible at the naked eye; this is why
you can usually see these inclusions on the screen.
The Polariscope, here a pocket polariscope. It is the usual means by
which one differentiates on the one hand the unirefractives
substances (amorphous or coming from the cubic system) and the
birefractives substances, coming from the other systems of
crystallization. This instrument is composed of two filters
polarizing between which the stone is placed, and it is while
turning the filter of the top that the analysis can be done.
The refractometer is used to determine the indexes of refraction of
the stone. This instrument calculates speed with which luminous
rays arise from the stone and indicates it on a scale. The gem
is placed on a coated pane of a fine particle of liquid made up
amongst other things of methylene iodide which shows an index of
refraction of 1.81 (this liquid is toxic)
Dichroscope of which
the principal part is made up of a tube comprising a window with
the one of the ends and a lens with the other, between the two
is assembled a piece of calcite, chosen because of its strong
double refraction so that two images of the window appear side
by side to the observer which looks in the eye-glass. The light
of the adjacent images of the window vibrates in two
perpendicular plans one to the other at a rate of one only plan
for each image.
The rotation of the stone is a significant
factor during the control of dichroism, bus in all the double
refraction stones, it exists one or two directions of
unirefringence, known as optical axe.
The Chelsea color filter.
There is a very effective means to reveal the subjacent
differences in color, it is the use of colored filters. They are
especially effective in the differentiation of natural and
synthetic emerald. The optical fiber lamp is useful to
distinguish inclusions in rough stones.
Microscopes: the
binocular one, i.e. to double objective is best adapted for
gemology. The microscope is of first importance in the detection
of counterfeits. Indeed, the study of inclusions which the stone
can contain is a practical means to identify the gems while
explaining to a certain extent the conditions under which they
were formed in nature.
The spectroscope concerns an extremely
simple method which allow the identification of all the stones;
rough or polished not being able to be seen with the refractometer and which
even makes it possible to distinguish certain synthetic or glasses.
This method makes it possible to analyze the nature of the dye of the stone. The
way in which the spectroscope analyzes the various wavelengths of the light is
in theory very simple; that depends on the different degree of refraction to
which its subjected various rays of each color (wavelength) when they are
propagated through a prism of glass or another transparent substance.
After having passed through a prism, a narrow parallel beam of white light is
spread out in a ribbon with the colors of the rainbow, the visible spectrum.
Let us note that a powerful instrument used in many fields of science is at
disposal of the gemologists, it is the microscope of Raman which spectroscopy
supplements the traditional technology of the laboratory. This instrument can
give information on samples lower than 1µm what is not accessible to any
technique.
The use of the Raman spectroscopy by imagery is single in order to determine in
a nondestructive way the molecular composition of inclusions present in the
gems.
Analyze by fluorescence: four kinds of radiation are used. The first consists
simply in passing a powerful luminous ray of a lamp of projection of 500W
through a balloon filled with a strong copper sulfate solution, this last
absorbs all the red light, orange and yellow and the filter rays are invisible
if one observes through a good red or orange filter. If a substance only lit by
the beam of blue light in which it is shines while being observed through one of
these filters, it emits a fluorescence.
The second method of stimulation of fluorescence is the used, it uses a lamp
with high pressure out of quartz, and whose light is filtered through the glass
of Wood.
The third uses the ultraviolet radiations of short radiation
emitted by a quartz-mercury lamp with low pressure, in which the
mercury line with 2537 Å is dominating.
X-rays represent the fourth source of radiation, they are
dangerous unless the source is not seriously armored. The unit
weight and its measurement. It frequently occurs that only an
approximate value of the density of a stone is necessary to
differentiate between gems from similar appearance; in this case
a control from the density can be carried out by means of
liquors of density.
The principle of the method is simple. A
stone will be inserted in a less dense liquid, will float in a
denser fluid and will remains in suspension in a liquid of equal
density.
The three liquids are:
- The bromoforme density 2,9.
- The methylene iodide density 3,33.
- The liquor of Clerici density 4,15.
PRINCIPAL COLORING ELEMENTS IN THE GEMS.
Chromium Ruby,
Emerald, Pyrope garnet, grossularite and uvarovite, Tourmaline.
Copper Dioptase, Malachite, Azurite. Iron Sapphire, Aquamarine,
Citrine, Almandin garnet. Manganese Morganite, Pink tourmaline,
Spessartite garnet. Nickel Chrysoprase. Titanium Blue Sapphire.
Vanadium Green beryl, Blue zoïsite , Garnet. GGGEMS.COM
© 1987 - 2004 gggems.com All rights reserved.
About the author:
http://www.gggems.com/gemology.htm
Back to Articles Contents Page
|
Articles