REMARKABLE DIAMOND
FACTS: 
- All diamonds are at
least 990,000,000 years old. Many are
3,200,000,000 years old (3.2 billion years)
- Diamonds are formed
deep within the Earth: between 100 km and 200 km below the
surface.
Diamonds form under remarkable conditions!
- The temperatures are
about 900 - 1300 C in the part of the Earth's mantle where
diamonds form
- The pressure is
between 45 - 60 kilobars (kB)
- 50 kB = 150 km = 90
miles below the surface
- 60 kB = 200 km =
120 miles below the surface
- Diamonds are carried
to the surface by volcanic eruptions.
The volcanic magma
conduit is known as a kimberlite pipe or diamond pipe. We find
diamonds as inclusions in the (rather ordinary looking) volcanic
rock known as kimberlite.
NOTE: The
kimberlite magmas
that carry diamonds to the surface are often much younger than the
diamonds they transport (the kimberlite magma simply acts as a
conveyer belt!).
- Diamond is made of
carbon (C), yet the stable form (polymorph) of carbon at the
Earth's surface is graphite.
- To ensure they are
not converted to graphite, diamonds must be transported extremely
rapidly to the Earth's surface.
It
is probable that kimberlite lavas carrying diamonds erupt at between
10 and 30 km/hour (Eggler, 1989). Within the last few kilometers,
the eruption velocity probably increases to several hundred km/hr
(supersonic!).
- Diamond is the
hardest material
Diamond is the
hardest gem on MOHS harness scale and graphite (also made from carbon
atoms) is the softest !! Given that both diamond and graphite are
made of carbon, this may seem surprising.
The explanation is
found in the fact that in diamond the carbon atoms are linked
together into a three-dimensional network whereas
in graphite, the carbon atoms are linked
into sheets with very little to
hold the sheets together (thus the sheets slide past each other
easily, making a very soft material).
Diamonds occur in two general types of deposits world
wide:
- volcanic pipes, also known as
kimberlite pipes
- alluvial, or placer,
deposits, which were formed by the erosion of diamond pipes over
millions of years.
The earliest productive mines were in the Golconda
region of India, particularly along the Kristna River. After 1725
this mining district was gradually eclipsed in importance by the
diamond deposits of Brazil. Diamonds were first mined there along
the Jequitinhonha River, in the Diamantina area of the state of
Minas Gerais.
In 1867 a 21-carat stone was discovered on the banks of
the Orange River near Hopetown, South Africa. A great diamond rush
started, and new deposits were discovered that were more productive
than any the world had ever known. Another major diamond resource
was developed in the 1950s in the Yakutia region of the Soviet
Union. By the 1980s the Yakutia and South African regions and the
country of Zaire dominated the world's diamond market. The mineral
has also been found in smaller amounts in numerous other places. In
the United States the leading producers include Arizona, Nevada, and
Montana, although the largest gemstones have been found in an eroded
volcanic pipe in Pike County, Ark.
For many years, microscopic diamonds have
occasionally been noted in meteorites; they were attributed to
high-speed collisions in space or with the Earth. In 1987, however,
following the discovery of many more such diamonds, the theory was
developed that they are the product of ancient supernova explosions
of giant stars.
In recent years, diamonds have been found in unusual
metamorphic rocks that were subjected to very high temperatures and
pressures.
HOW RARE ARE DIAMONDS?
· How many grams do you need
to mine to get 5 grams of diamonds?
(5g/1000 kg) @ 1000 g/kg = 5 g /1,000,000
g!
BUT only 20 % are gem quality (80 % of these are sold
in a 'managed selling environment') and the remainder are used for
industrial purposes (this material is known as 'bort' or 'carbonado'
(carbonado is finer)).
CHARACTERISTICS OF
DIAMOND:
· Hardness = 10
· Crystal System = cubic
· This is what crystals look
like before they are faceted: note their natural octahedral
shape!
Uncut diamonds are
also found in cubic
forms
· Diamond has four good
cleavages, thus diamonds tend to cleave on impact
· Other
diagnostic
properties.
Other issues: Treatment, simulants,
synthetics
(1) TREATMENTS:
Surface cracks and cleavages reaching the surface: often
with a glass-like material
Identification: optical microscope examination:
- =greasy appearance
- =flash effect
- =bubbles
Problem: Filling does not always resist polishing and
cleaning
- (b) drilling of
inclusions
Solutions can be poured into the resulting "hair-width"
diameter hole to bleach colored inclusions.
Irradiation is used
to change the color of the diamond. A common color produced by
irradiation is green.
Early attempts: beginning of 20th Century: diamonds
exposed to radium - the problem was that the diamonds remained
radioactive!! However, modern irradiation treatments do not produce
radioactive stones.
Irradiation
involves the use of devices such as:
o
(1)
linear accelerators
o
(2)
gamma ray facilities
o
(3)
nuclear reactors
Detection of irradiation treatment:
Electron
irradiation only changes the surface of the stone. Thus, it produces
a concentration
of color where the gemstone
is thin. For example, electron irradiation produces a color
concentration at the culet or keel line of the faceted gem
(2) SIMULANTS
Simulants - simulate the appearance of diamond
The distinction between a synthetic diamond (man-made
diamond consisting of carbon atoms arranged in the typical diamond
structure) and a diamond simulant (not a carbon compound with the
diamond structure) is VERY important!!
In order of increasing R.I., the most common simulants
are:
- YAG
= yttrium aluminum garnet
- GGG
= gadolinium gallium garnet
- CZ =
cubic zirconia
- Strontium titanate
- diamond.
This rhyme can be used to memorize the common diamond
simulants in the above order:
You
go crazy staring at diamonds.
Again: Simulants (look alsikes) differ from synthetics
(synthesized by humans!) !!!!
Simulants are
distinguished from diamonds using measurement or observation of
various properties, such as:
- R.I.
- "Read through effect"
- Dispersion
- Hardness
- Specific Gravity
- Reflection pattern
- Shadow
patterns
Note: not all
diamond simulants have been around for the same length of
time!
(3) SYNTHESIS
(Details
on gem synthesis)
Synthetic diamonds
are often yellowish in color
(rarely used for gem purposes, more commonly used as diamond grit
for industrial purposes. Modern synthesis of thin film diamond has
other industrial applications).
A 5 mm diamond (0.5 carat) takes over a week to grow.
Synthesis requires:
Synthetic diamonds
can sometimes be distinguished from natural diamonds by the presence
of flux
inclusions (Ni, Al or
Fe).
