Mineral Jewelry Rocks Organic Amber Jet
Mineral Jewelry Rocks Organic Amber Jet
Mineral Jewelry Rocks Organic Amber Jet
TYPES OF STONES
All the gem minerals are grouped into different species according
to their chemical composition, and each species is subdivided into
varieties in relation to colour or any other special optical
characteristics. Some gemstone species are collectively described
into families or groups, especially according to their similar
crystalline character.
Gems and stones are classified into many groups:-
Precious stones
Gemstones which are highly valuable for their hardness and
rarity are known as Precious Stones. Precious Stones are
generally expensive in comparison of Semi-Precious Stones.
There are only four Precious Stones:
1. Diamond
Diamonds are considered as precious gem. It is regarded as the 'king of the
gems'. Chemically, diamond is a crystalline isotrophe of carbon and with a
hardness of 10, it is the hardest known substance on earth.
It has a specific density of 3.15 to 3.55 and a refractive index of 2.5.
Incredibly, diamond is the only gemstone made of just one element carbon.
2.
2. Ruby
3. Sapphire
Color - Blue, Yellow, Green, White, Colorless, Pink, Orange, Brown and
Purple
Mohs hardness scale - 9
Mineral Class - Corundum
Source - Found mainly in Sri Lanka, Thailand, Burma, Australia, India,
Brazil and Africa
Sapphire is also a variety of the mineral Corundum and represents all the colors
except red Corundum, which is Ruby. Its physical and chemical properties are
virtually similar to properties of Ruby. Blue is the main color of the Sapphire
whereas this gemstone is also found in colors like green, orange, pink, gray,
colorless, black, brown, and purple.
Sapphire is the most precious blue gemstone. It is a most desirable gem due to
its color, hardness, durability, and luster. Value of this gemstone depends on its
size, color and transparency. Top-quality sapphires are extremely rare in all the
gemstone mines of the world. Cutting of this gemstone requires great skills and
experience and it is the job of the cutter to orientate the raw crystals in such a
way that the color is brought out to its best improvement. Both Sapphires and
Rubies have been successfully and widely produced synthetically in laboratories,
and in appearance, chemical composition and hardness are almost identical to
the natural gems. France is the major production hub for synthetic Corundum
4. Emerald
Semi-Precious stones
Gemstones those are valued for their beauty and not covered
under any one of the four Precious Stones, Diamond,
Emerald, Ruby or Sapphire, are known as Semi-Precious
Stones. Semi-Precious Stones are available in all price ranges
from low priced to high priced. There are many SemiPrecious Stones:
Alexandrite
Garnet
Amethyst
Opal
Aquamarine
Pearl
EXAMINATION OF GEMSTONES
Physical
General Appearance
The general appearance is studied by simply taking the
gemstone in hand and observing it carefully. You should also
feel the texture of the stone's surface for initial examination.
What you might consider as a gemstone can be a piece of
shiny rock or mineral. Gemstones are clear with a smooth
texture. If the stone surface is rough and feels like sand, it
may not be a precious stone.
Color
Examining the color to identify a gemstone. Stones
characterized as colorless should not have any tinge of color
in them, while colored stones should be dark and deep
colored. Darker the color, better will be the quality. But color
alone should not be a property to study because many times
color can lead to wrong identification. For example, sapphire
is generally considered to be blue in color, but only few know
that this stone is also found in shades of green, yellow, and
pink.
Lustre
It is the appearance of a material surface in reflected light. It
is best assessed where the polish is as fine as possible. A
Dispersion
It is the ability to split white light into its basic spectral
colours (the colours of the rainbow). In gemstones, this is
translated into flashes of colour when the stone or the light
source moves. Not all gemstones exhibit this property.
Diamond is famous for its fire, which is the term given to the
appearance of its moderate, yet highly noticeable, dispersion.
Garnets such as andradite, when light enough, also show
moderate dispersion. Ruby and sapphire, on the other hand,
exhibit a weaker dispersion which is not normally visible.
Carat Size
The purity of a gemstone is determined by its carat size
wherein, one carat is equal to 100 cents. More the carat size,
larger is the stone and better is its purity. The value of a
gemstone is directly proportional to its carat size. More the
carat size, greater is its value and vice-versa.
Crystal Shape
The shape of a crystal is due to its physical structure. Every
gemstone has a peculiar shape owing to its molecular
structure and atomic number. This is a crucial examination
Chemical
A mineral is usually described by a chemical formula that indicates
the type and amount of its major chemical elements. The chemical
formula is a simplified representation of the composition of the
stone and usually contains the main elements only. However, there
are other chemical compounds present in a stone in smaller
quantities: these are the trace elements. In gems, trace elements are
often the agents responsible for adding colour. Despite their small
contribution to the overall chemical composition of the gemstone,
trace elements play a key role in determining whether a mineral is
just a standard stone or a rare and coveted gemstone. By means of
chemical analysis, gem labs can identify and quantify the presence
of such trace elements, which also give valuable hints as to the
geographic origin of a gem.
Aside from the main and trace elements, gemstones also contain a
very high number of other elements in minute concentrations of a
few parts per million or billion. These so-called ultra-trace
elements typically do not have any significant influence on the
appearance of the gemstone, but they shed light on the
environment in which it grew thousands, millions or even billions
of years ago. The type and amount of these elements in a gemstone
are often indicative of a specific location and are used by gem labs
to determine its country of origin.
Chemical Classifications
Chemical Class
Silicates
An Example
Tourmaline, (Mg,Fe)2 SiO4
Oxides
Oxygen
Corundum, Al2O3
Carbonates
Rhodochrosite, MnCO3
Native Elements
Diamond, C
Sulfides
Sulfur
Sphalerite, ZnS
Halides
Fluorite, CaF2
Phosphates
Sulfates
Sphalerite
Fluorite
Gypsum
Rhodochrosite
Apatite
Corundum
Chemical Bonding
The forces that bind atoms, ions, or ionic groups together in crystalline
solids are electrical. The force type and intensity are responsible for the
physical and chemical properties of minerals. The stronger the bond the
harder the crystal, and higher the melting point. The high hardness of
diamond is because of the strong electrical bonding forces linking the
carbon atoms. These electrical forces holding inorganic minerals
together are chemical bonds, such as: ionic, covalent, van der Waals,
metallic, hydrogen, or some combination.
CORAL
Like pearl, Coral also is not a true stone. In fact it is fossilized
from a tiny sea creature known as corals.
Coral is worn to please the Mars. Heat and light of the sun play
a major role in the formation of coral.
Its specific density is 2.65; refractive index 1.486 to 1.66 and
hardness is 4.0. Coral reacts with hydrochloric acid to form
froth.
When touched by a hot wire coral smells like burning hair
PEARL
RUBY
This is the stone of the sun and hence has a supreme position
among the Navrartnas.
BLUE SAPPHIRE
It is the most talked about gem that belongs to Navratna
category. This beautiful attractive and transparent gem is the
main stone of Saturn and has tremendous powers.
Basically, sapphire, topaz and ruby belong to same class of
stones.
A slight variation in colour differentiates them from one another.
Aluminum oxide is the main constituent of blue sapphire.
In hardness, specific density and refractive index sapphire is
similar to topaz and ruby.
Analytical
Refractive Index (RI)
The refractive index (RI) of a material is the ratio of the speed of
light in air to the speed of light in the material. It affects the
brilliance of a stone by determining the maximum angle at which
light reflects on its surface instead of refracting into the stone. This,
directions within the stone. In the first case, the stone would be
singly refractive (SR) or isotropic, while the latter would be doubly
refractive (DR) or anisotropic. These properties greatly affect how
the mineral interacts with light and the appearance of colours.
The polariscope is an instrument that helps to identify this optical
character. It consists of a basic combination of a light source and
two rotating polarising filters. The stone is placed in between the
filters with the light shining through the filters and the stone from
underneath. The stone and the upper filter are then rotated
horizontally in various positions to assess the optical nature of the
stone. This test, though not always conclusive, is another vector
that assists in the gem identification process.
Dichroscope
The dichroscope is a small handheld instrument enclosing a calcite
prism. It is used by shining a narrow beam of light through the
stone and observing the light coming out of the stone into the
dichroscope.This device enables to separate light rays travelling in
different directions. Effectively, this shows the pleochroic colours,
when present, which make up the colour of light transmitted in
one direction through the crystal. Singly refractive (SR) minerals,
within which light travels as a single ray, will only ever display
one colour through the dichroscope, regardless of the direction
light passes through the crystal; whereas doubly refractive (DR)
minerals, which split light in two different rays, will display two
different colours through the dichroscope. When those two colours
remain the same no matter what direction light passes through the
crystal, it implies that the stone is DR uniaxial, that is, it has one
optic axis. When a third colour is seen with one of the other
previous two in a different crystal direction (usually best observed
at 90 to the first direction), it reveals that the stone is biaxial,
which means it has two optic axes.Some gemstones have very
similar properties, yet can sometimes be differentiated on the basis
of their optic character.
Spectroscope
It is sometimes possible to estimate the identity of a stone quite
accurately by visual observation only. This is explained partly by
the peculiar shades of colour recognizable in certain gem species.
These can be so unique that they may be distinguished with ease
amongst others.
However, there are many situations when colours will appear so
much alike others that it is impossible to distinguish one gem from
another. For example let us consider a brownish-orange
spessartine garnet against an almandine garnet of the same colour.
If the colours are similar enough, it is virtually impossible to tell
them apart. Naturally, other gemmological tests will reveal almost
identical RI and SG, as well the same singly refractive optical
nature. Although the human eye cannot detect subtle differences
between the colours of those garnet species, it does not mean none
exists. This is where the spectroscope comes in as valuable
assistance. This device enables the observer to view the entire
visible spectrum for those colours, i.e. the splitting of the visible
light into the spectral colours of the rainbow. The visible spectra of
the two garnets would show similarities since the stones are both
garnets of the same colour, yet there will also be marked
differences related to the nature of each species. In this case,
spessartine garnet will display absorption lines (lines of darkness)
at the frequencies of 412, 424 and 432 nm, located in the violet part
of the visible spectrum. These absorption lines, related to the
presence of manganese, will not be present in the visible spectrum
of almandine garnet, which does not contain manganese.
Microscope
The microscope remains an essential tool in gemmology.
Observation of the internal characteristics of gemstones remains
one of the safest and most valuable techniques in the recognition of
the nature of precious minerals. Certain features are so typical of
some varieties that they not only help identify the mineral, but
even point to the geographical origin of a sample.
SPECIFIC GRAVITY
( Hydrostatic method)
REFRACTIVE INDEX
(Refactometer Range
1.30-1.80)
OTHER CHARACTERISTICS
Chatoyancy and iridescence
(visual) surface irregularities
(10 *Loupe) special adsorption
spectral signature (table
spectrometer), magnetism
(magnet), thermal conductivity
(thermal probe), colour
absorption (Chelsea filter,
fibrolite (biaxial/uniaxial
character (polariscope),
Diamond weight and size
(Mohs diamond gauze)
(1 Carat = 200mg
SHAPE
Electronic Balance)
LUSTURE
(Visual Estimation)
PLEOCHROISM
(Dichroscope,
microscope)
LUMINISCENCE
(lw-365nm and sw253.7nm UV Lamps)
NAMING OF
MINERAL / GEMSTONE
TRANSPARENCY
(10 *Loupe)
GROWTH LINES
(10*Loupe,Gemolite
Microscope)
INCLUSION
(10*Loupe,Gemolite
Microscope)
Microscope
Dichroscope
THEIR USES
Spectroscope
The spectroscope is sometimes used to separate
natural from synthetic gem materials, as variations
in chemical composition can be revealed in the
absorption spectrum of light transmitted through
the stone.
Refractometer
Used to differentiate gemstones that have similar
colors since gemstones can have other optical
properties that make them different from others.
Gem Refractometers are also used for identifying
the Refractive Index of Gemstones
Polariscope
Chelsea Filters
1.
2.
3.
4.
5.
6.
7.
Color: blue
Transparency: transparent
Optic Character: DR
Luster: vitreous
RI high: 1.583
Magnification: hollow growth tubes
RI Low: 1.569
The possible gem is aquamarine.
INDEX
1. Introduction .
2. Types of stones .
Precious and Semi-Precious .
Diamonds and coloured stones
Natural and Man-Made
Organic and Inorganic
Amorphous and Crystalline
3. Examination of Gemstones
Physical
- General Appearance .
- Colour
- Lusture .
- Dispersion .
- Carat Size ..
- Crystal shape .
- Transparency .
- Streak Test
- Hardness ..
- Cleavage
- Loupe
- Pleochorism ..
Chemical
- Chemical Bonding .
- Chemical composition of
various Precious stones
Analytical .
- Refractive Index ..
- Polariscope
- Spectroscope
- Microscope .
- Dichroscope ..
- Spectophotometry ..
- Spectrometry
UV-IF Spectrometry .
Multi-Channel Spectrometry .
Raman Spectrometry
FT-IR Spectrometry
Fluorescence (UV Lamps)
4. Flowchart depicting Identification of Stones .
5. High-Tech Machinery used in analysis of gemstones .
6. Example-Analysis of a Gemstone
GEMSTOnE anaLYSiS
arpiTa Jain