Unique Properties of Diamonds
Diamond is carbon in its most concentrated form. Except
for trace impurities, diamond is composed solely of carbon, the chemical
element that is fundamental to all life. Yet diamond is distinctly
different from its close cousin: graphite, which is also composed solely
of carbon. The particular arrangement of carbon atoms or its crystal
structure - the feature that defines any mineral's fundamental properties
- is what makes diamond so brilliant, refractive and durable.
A neutral carbon atom has six protons and six electrons
surrounding its nucleus. Four of the electrons in a carbon atom are
valence electrons, which are electrons that are available to form bonds
with other atoms. In graphite, each carbon atom bonds only three of its
four valence electrons with neighbouring carbons. In diamond however,
every carbon shares all four of its available electrons with adjacent
carbon atoms, forming a tetrahedral unit. This shared electron-pair
bonding forms the strongest known chemical linkage, the covalent bond,
which is responsible for many of diamond's superlative properties. The
repeating structural unit of diamond consists of eight atoms that are
fundamentally arranged in a cube.
Hardness
The only natural substance that can scratch a diamond is another diamond.
Hardness is the measure of a substance's resistance to being scratched,
meaning diamond is the hardest mineral known.
Resistance to Fracture
Hardness is not the only unique characteristic of a mineral's durability -
the relative resistance to fracture is another. Although diamond is not
fragile or prone to breaking apart, all substances including diamond can
fracture or shatter. Due to its particular crystal structure, diamond has
certain planes of weakness along which it can be split. Diamond is said to
have perfect cleavage in four different directions, meaning it will
separate neatly along these lines rather than in a jagged or irregular
fashion. Diamond cutters take advantage of cleavage to fashion diamonds
efficiently.
Density
Density is a ratio of a substance's mass to its volume, and diamond is
amazingly dense given the low atomic weight of carbon. The fact that
diamond is much more dense than graphite offers an important clue as to
diamond's origin: the fact that diamond's carbon atoms are "squeezed"
together tighter than in graphite implies that diamond is formed under
high-pressure conditions.
Brilliance and Lustre
Diamond's brilliance and lustre are two of its most valued attributes. The
science behind such phenomena is diamond's great ability to refract light;
that is, to bend or slow light as it passes through it. The amount that a
substance can impact light in these ways is quantified in its refractive
index. Generally speaking, higher density materials have greater
concentrations of electrons and therefore greater capabilities to refract
light. Light passing through diamond is reduced to only about 77,000 miles
per second, which is near the maximum for any transparent substance.
Refraction
The refractive index can also be used to describe how visible light can be
split into the colours of the spectrum when passing through diamond.
Essentially, this happens because the refractive index of a substance is
not constant, but rather varies for different wavelengths, or colours, of
light. Consequently, the shorter wavelengths of light (the blue end of the
spectrum) are bent more than the longer wavelengths (the red) when
entering a diamond at an angle. Thus, the colours separate, or disperse,
producing the visible spectrum as from a prism.
Reflectance
Reflectance, or the amount of light reflected from a transparent
substance, can also be inferred from a material's refractive index. Once
again, diamond displays the maximum amount of reflectance for a
transparent substance, displaying what is called an "adamantine" lustre.
Colour
Our standard conception of diamond is as a colourless stone. But colour in
diamond exists in myriad variations, from dazzling pinks and yellows to
blues and violet. A chemically pure, perfect crystal of diamond is
colourless, but add a little nitrogen and yellow appears. Add boron
instead and a blue diamond results. Colours from red to violet, real
white, and black are also possible, and coloured diamonds are highly
desirable.
Fluorescence and Phosphorescence
An interesting property of some diamonds are fluorescent in that they can
glow in the dark. When illuminated by ultraviolet light, certain diamonds
can absorb the high-energy radiation and re-emit it as visible light. Some
diamonds can even continue glowing after the ultraviolet source is turned
off, and are called phosphorescent.
Conduction
Diamonds are called "ice" with good reason. Objects feel cold not only
because they are at a lower temperature than our bodies, but also because
they can extract or conduct the heat away from us. When you touch a
diamond to your lips, it feels cold because it robs your lips of their
heat. The capacity of diamond to conduct heat distinguishes it readily
from other gems and exceeds that of copper, an excellent thermal
conductor, by about four times at room temperature. This exceptional
property of diamond is increasingly being used for extracting heat from
electronic devices to make them smaller and more powerful.
Where do diamonds come from?
Experiments and the high density of diamonds tell us that
they crystallize at very high pressures. In nature this means that
geologic processes at great depth within Earth create diamonds, generally
more than 150 kilometres beneath the surface in a region beneath the crust
known as the mantle.
Diamonds ascend to the Earth's surface in rare molten
rock, or magma, that originates at great depths. Carrying diamonds and
other samples from Earth's mantle, this magma rises and erupts in small
but violent volcanoes. Just beneath such volcanoes is a carrot-shaped
"pipe" filled with volcanic rock, mantle fragments, and some embedded
diamonds. The rock is called kimberlite after the city of Kimberley, South
Africa, where the pipes were first discovered in the 1870s. Another rock
that provides diamonds is lamproite.
The volcano that carries diamond to the surface emanates
from deep cracks and fissures called dykes. It develops its carrot shape
near the surface, when gases separate from the magma, and a violent
supersonic eruption follows. The volcanic cone formed above the kimberlite
pipe is very small in comparison with volcanoes, but the magma originates
at depths at least three times as great. These deep roots enable
kimberlite to tap the source of diamonds.
Diamonds are found on continental cores
The search for diamonds has determined that most are
derived from kimberlite pipes in the oldest, nuclear portions of the
continents. The oldest parts of the continents are called cratons, of
which there are two types: Archean-age archons (older than 2,500 million
years) and Proterozoic-age protons (1,600 to 2,500 million years old).
Kimberlite pipes occur in many parts of the continental crust, but most
diamond-rich ones are found in archons.
Indicator minerals
Certain minerals are present in the rocks from the upper
mantle that occur with diamonds in kimberlite and lamproite pipes. Some of
these minerals, being resistant to weathering and denser than quartz sand,
concentrate in channel bottoms. Because they occur in far greater
abundance than diamond, exploration geologists look for these "indicators"
among the gravel of regions they suspect may host diamond-bearing pipes.
Indicator minerals for diamond include, in order of
decreasing significance: garnet, chromite, ilmenite, clinopyroxene,
olivine, and zircon.
Kimberlite and lamproite
The complex volcanic magmas that solidify into kimberlite
and lamproite are not the source of diamonds, only the elevators that
bring them with other minerals and mantle rocks to Earth's surface.
Kimberlite and lamproite are similar mixtures of rock material whose
important constituents include fragments of rock from Earth's mantle,
large crystals, and the crystallized magma that glues the mixture
together. The magmas are very rich in magnesium and volatile compounds
such as water and carbon dioxide, and as the volatiles change to gas near
Earth's surface, explosive eruptions create the characteristic carrot or
bowl-shaped pipes. Kimberlite magma rises through Earth's crust in
networks of cracks or dykes - the pipes only form near Earth's surface.
Kimberlites are generally much younger than the diamonds
they bring to Earth's surface. They have been dated between 50 and 1,600
million years old, whereas diamonds range from about 3.3 billion years old
to less than one billion years old. These age differences help clarify a
picture of diamonds having crystallized and been stored beneath the
ancient continental cratons and only later being lifted to Earth's surface
by kimberlites.
Xenoliths
Kimberlite magmas carry foreign rocks called xenoliths
from Earth's mantle to the surface. Xenoliths are geologists' only samples
from the deep Earth, and carry information about diamond growth
conditions. The two most common types of xenoliths are peridotites and
eclogites. Peridotite is the main constituent of the mantle beneath the
crust and consists primarily of olivine. Eclogite, a very different rock
consisting primarily of garnet and a green pyroxene, is formed by plate
tectonics when basalt of the ocean crust founders into the mantle, may
also contain some diamonds.
Diamond inclusions
Diamonds with inclusions are like little space capsules
from the mantle: pristine mineral samples are protected by the diamond's
indomitable embrace and transported to the surface by a volcanic rocket.
Inclusions capture a picture of the rock and environment in which diamonds
grow and indicate that garnet harzburgite and eclogite are the most common
rocks in which diamonds have grown.
A single mineral inclusion rarely defines a specific rock,
but two or more minerals may enable interpretation of rock associations
and origin. Certain inclusion minerals are virtually unique to diamond
sources and are thus sought in the exploration for diamonds.
Types of deposits
Geologic processes create two basic types of diamond
deposits, referred to as primary and secondary sources. Primary sources
are the kimberlite and lamproite pipes that raise diamonds from Earth's
mantle, where they originate. Secondary sources, created by erosion,
include such deposits as surface scatterings around a pipe, concentrations
in river channels, and fluxes from rivers moved by wave action along ocean
coasts, past and present.
Mining a pipe
Mining of a diamond-bearing pipe starts with the
excavation of a pit into the pipe. In this process, called "open-pit" or
"open-cast" mining, the initially loose and eventually hard ore material
is removed with large hydraulic shovels and ore trucks. Hard rock is
drilled and blasted with explosives so the broken material can be removed.
When deep, rich ore warrants it, the mining goes underground with vertical
shafts descending to horizontal drifts, or passageways that enter the
pipe.
Processing diamond ore
Once a mining operation yields ore, the diamonds must be
sorted from the other materials. This process relies primarily on
diamond's high density. An old but effective method is to use a washing
pan, which forces heavy minerals like diamond to the bottom and waste to
the top. Cones and cyclones use swirling heavy fluids mixed with crushed
ore to achieve density separations. Once 99 percent of the waste in the
ore removed, further separations may use either a grease table or an x-ray
separator. Final separation and sorting is done by eye.
Alluvial mining
Most of the diamond deposits first discovered were
alluvial - concentrations in streambed or riverbed sand and gravel. They
are still actively exploited in many ways, from the most primitive to the
highly sophisticated. The goal is relatively simple: to find a location
where moving water has deposited diamonds in the bottom of a channel,
possibly in a pocket or cleft. Because rivers meander and drainage can
change, the search for alluvial diamonds requires some geological
knowledge and a lot of luck. The process involves removing the overlying
barren ground, digging up the bearing ground, extracting the diamonds,
and, nowadays, restoring the landscape when finished.
Conflict diamonds
Conflict diamonds are diamonds that originate from areas
controlled by forces or factions opposed to legitimate and internationally
recognized governments, and are used to fund military action in opposition
to those governments. In 2000, the United Nations General Assembly adopted
the "Kimberly Process" to stem the flow of conflict diamonds, protect the
legitimate diamond industry and create and implement an international
certification scheme for rough diamonds. In July 2000, the World
Federation of Diamond Bourses and the International Diamond Manufacturers
Association created the World Diamond Council to assist governments in
creating a system of certified, non-conflict diamonds.
