Pink Diamonds of the Argyle Mine
Karen Rice (Gemologist for Suna Brothers, Inc., New
York, New York, USA)
2004 Central Ohio Mineral, Fossil, Gem & Jewelry
Show (Columbus, Ohio, USA)
28 March 2004
Pink
diamonds are rare & beautiful. Diamonds come in almost literally
any color.
Yellow
is the most common colored diamond (canaries); yellow color is caused by a few
ppm of nitrogen. Examples of blue include the famous Hope Diamond - the
most visited specimen in the world. The Hope Diamond is 45.5 carats large.
Most blues are not Hope-like. Most blues are actually a pale blue.
The coloring agent in blue diamonds is a few ppm of boron. Traditionally,
all blue diamonds were thought to be 2-B diamonds (they conduct
electricity). Now, it is recognized that a few blue diamonds are not type
2-B. A few type 1-A blue diamonds are known from the Argyle Mine of
Australia. Type 1-A diamonds are insulators (they don’t conduct
electricity).
“Green”
- very rare. Natural radiation has caused the color - usually, get only a
green skin.
Red
- a very rare color. Many “reds” are certified as secondary colors, such
as pinkish red, purplish red, etc., much to the chagrin of their owners, as a
“red” certification boosts the value of the diamond considerably.
Eye-visible
inclusions are OK in colored diamonds.
Argyle
pink diamonds - in order to best explain these, it’s necessary to look back in
history.
In
1662, a large pink diamond from Golconda, India was observed & illustrated
in a publication - “The Great Table” diamond. It is possible that
the original “Great Table” diamond now exists in 2 cut pieces - they were part
of the Iranian treasury (sold to the Shah of Iran in 1958) before they
disappeared after the 1979 Islamic revolution in Iran.
South
Africa has produced pink diamonds - a famous one is from Tanzania (rough 54.5
carats & cut to 23 carats) - it is a delicate pink specimen.
The
Argyle Mine of Australia was found in 1979. The material from Argyle
forced a redefinition of what pink diamonds meant. Argyle Mine pink diamonds
are intensely pink, unlike the subtle pinks from India &
Africa. Kimberlite diatreme pipes were recognized in 1874 as the primary
source of diamonds. The Argyle Mine is over a pipe that was initially
identified as a kimberlite, but subsequent examination showed that it is a
lamproite. Kimberlites & lamproites are related, but are
distinct. Kimberlite pipes have a steep cone shape (~80° angle sides),
while lamproite pipes have a champaign glass shape (the top of the pipe
consists of a collapsed rock deposit).
In
Africa, diamond-bearing regions are in old cratonic areas, such as the West
African Craton, the Central African Craton, and the Kapvaal Craton (Africa’s
oldest craton, at 3.5 b.y., in southern Africa).
The
northern part of Western Australia State has the Kimberley Craton. The
Argyle Mine is not on the Kimberley Craton itself, oddly. The Argyle Mine
is located along a mobile zone (Halls Creek Mobile Zone), just off the eastern
edge of the Kimberley Craton. The Argyle Mine is mining the only economic
diamondiferous lamproite pipe in the world.
1851
- alluvial diamonds were found in New South Wales, southeastern Australia, in
the Bingara area.
1895
- diamonds discovered in the Nullagine area of Western Australia.
late
1960s & early 1970s - the Argyle area was explored. The Argyle Mine
itself was found in 1979.
The
Argyle Mine is now owned by Rio Tinto, a large mining organization. The
Eastern Kimberly region of Western Australia is 1200 miles northeast of Perth,
300 miles from Darwin, and 110 km south of Kununurra. The area is a
barren countryside with few roads & hot. It is a plateau with a few
hilly areas (Matsu Range). The Argyle Mine was discovered using indicator
minerals, a classic prospecting technique. The lamproite pipe was
discovered October 1979 - it is 125 acres in size. It is adjacent to a
ridge of quartzite, which protected it from erosion. Nearby pipes to the
south are eroded down to their root zones. It erupted 1.2 b.y. ago.
The lamproite pipe has an elongated shape, map-wise, partially due to lateral
faulting 800 m.y. ago that carried a slice of it southwest-ward.
Argyle
Mine diamonds are 1.6 b.y. old - 400 m.y. older than the pipe eruption.
The
lamproite tilts northward at 30°.
Two
principal facies - sandy tuff lamproite (50% quartz grains) & non-sandy
tuff lamproite (<5% quartz grains).
1983
- mining of diamondiferous secondary (alluvial) deposits started - modern &
ancient floodplains & alluvial diamonds downstream from the pipe, along
Smoke Creek & Limestone Creek. Lots of alluvial diamonds were found
in it (all within 6 miles of the pipe itself).
From
1983 to 1985, 17 million carats worth of diamonds were recovered from secondary
deposits (all alluvial). Then, lots of holes were drilled to define the pipe
itself. The southern part of the lamproite pipe was economic - produces 6
carats of diamonds per ton of rock - a very high grade diamond deposit.
1985
- mining of lamproite pipe began, as a terraced open pit mine.
1991
- a 42 carat stone was found at the Argyle Mine.
30,000
tons of crushed rock produces 40 tons of heavies with possible diamonds in
it. Diamonds are separated from the rest of the heavies (“sinks”) by
x-ray sorters. Dried sinks are passed by an x-ray source & any
fluorescent specimen is blown aside by an airjet into a diamond bin. 95%
of the material in the diamond bin are actually diamonds & 5% are other
minerals.
>40
million carats were produced from the Argyle Mine in 1994 alone.
Most
Argyle Mine diamonds (60%) are industrial quality. Many are
“near-gem”. About 5% of Argyle Mine diamonds are gem-quality.
Most
Argyle Mine diamonds are brown colored & shaped as dodecahedrons.
Argyle
brown diamonds have been successfully marketed as “cognac diamonds”.
Many
Argyle specimens are white (clear).
Only
~2% of Argyle diamonds are colored stones. Some blues (type 1A diamonds
here - blue due to presence of hydrogen), but not impressive. The real
prize is the pinks - of the 25-30 million carats produced a year now, ~10,000
rough carats are pinks (each year). Argyle is the only consistent source
of colored diamonds in the world.
The
browns are sent to India for cutting.
The
pinks are sent to Perth for cutting.
~50
pink stones of 0.5 to 1.5 carat size (suitable for a center stone) are produced
per year from Argyle.
There
is a large range of pink colors.
Pinks
are much more included than the blues here. About 7% of the Argyle pinks
grade out as flawless. At first, the pink coloration was thought to be
caused by the presence of manganese (Mn), but it is now thought to be possibly
related to heat/metamorphism-induced microfaulting/slippage of carbon atoms,
resulting in pink banding at a microscopic level.
