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Sedimentary Diamonds?


Has anyone ever heard of diamonds being formed in metamorphosed
C-carrying shale deposits (ie slate)?

In theory it would be possible.


Actually, no, it would not happen. All diamond genesis takes place
below the Mojorovicic Discontinuity, and there is not any shale

Any shale or slate that might get carries into a subcrustal
environment via a subduction zone would cease to exist as shale, and
the pressure and heat in that environment fall far short of that
needed for diamond genesis.

Sorry, wrong physics.

By definition, metamorphism is a crustal process, usually
orogenetically realted (but not always). The pressures and
temperatures again fall short.

Wayne Emery
The Gemcutter

Has anyone ever heard of diamonds being formed in metamorphosed
C-carrying shale deposits (ie slate)? In theory it would be

No it wouldn’t. Temperatures and pressures are not high enough.
Shales and slates are formed at shallower depths (ie, pressure), and
lower temps, than the upper mantle layers where diamonds formed to be
carried up by kimberlite pipes. Even common igneous rocks, some of
which might contain various carbon compounds, don’t give you
diamonds. Still to shallow a depth of formation. Kimberlites are
unique not only in the depth from which the kimberlites are brought
up, but as well, the explosive speeds at which the lava came up,
which didn’t allow enough time for the diamonds to be destroyed on
the way up before it cooled enough so the crystals would remain
stable. To date, so far as I know, the only significant sources of
diamond are kimberlites, or diamonds weathered out of them. Oh, that
and impact formed ones from meteorite strikes. Those, when they
occur, do so such that even a small rock may have them in the
millions, but you need a powerful microscope to see them. Extremly
tiny nano sized diamonds…

Peter Rowe


Dont think it is possible, the temperature is too low and the high
water content of the rock would form carbonates instead. There are
also oxygen fugacity considerations as well.

Nick royall



Has anyone ever heard of diamonds being formed in metamorphosed
C-carrying shale deposits (ie slate)? 

Is it possible they were not truly formed there, but deposited
there as wash-downs, then integrated into the shale?



Hi Peter,

Kimberlites are certainly the predominant source of diamond host
rock, but lamproites provide diamonds, too, in some instances. Both
the Argyle mine in Australia and the diamond deposits of
Murfreesboro, Arkansas (Crater of Diamonds) are lamproite hosts.

Of course, there are all sorts of secondary diamond deposits such as
the diamond-beairng conglomerates of central West Africa and the
alluvial deposits in the rivers and ocean off Botswana, where the
diamonds have eroded out of those conglomerates.

But, as you note, shale or slate is a low pressure, low temperature
product which cannot support diamond genesis regardless of the
amount of carbon present.

Wayne Emery
The Gemcutter

Has anyone ever heard of diamonds being formed in metamorphosed
C-carrying shale deposits (ie slate)? In theory it would be

Whose theory? Think about it. Slate is a low-grade metamorphic rock.
As heat and pressure increase, slate becomes phyllite, then schist
with still more heat, and eventually as it melts and recrystallizes
it turns into gneiss or even a granite-gneiss. Since diamonds require
the temperatures/pressure of the mantle, a slate carried to that
depth by subduction would no longer be “slate.” There is a rock known
as eclogite that is formed in the upper mantle and has been
demonstrated to sometimes carry diamonds. Eclogite is very similar
to ordinary schists and gneisses, but the mineralization is totally
different. Perhaps eclogite was once a carbon bearing sedimentary
rock that was originally formed under near surface conditions. There
might be a doctoral thesis lurking in there somewhere!

Incidentally, the diamonds found in Brazil were in a sedimentary
rock, a flexible sandstone, but they didn’t form there.

Dick Davies


No, it would not be possible. Slate is a metamorphic rock that
originates with shale type sedimentary rocks that are made of clay or
composed of volcanic ash.

Diamonds are found in kimberlite, an igneous rock that is found in
pipes that originate between 150 and 450 km below the earth’s surface
in the mantle, deeper than other known igneous rock.

Mike DeBurgh, GJG
Henderson, NV

Is it possible they were not truly *formed* there, but deposited
there as wash-downs, then integrated into the shale? 

I’d say so, yes. Diamonds washed out of the original deposit
(alluvial diamonds) could end up in any sort of silt or sand sediment
layer, perhaps later to be formed into a sedimentary rock, and
perhaps later still, exposed to conditions forming metamorphic rocks.
However, the metamorphic rocks normally are altered over considerable
time, and the processes necessarily involve a good deal of heat and
pressure. It would also be possible for those same processes to
destroy the diamond crystals in the sediment if conditions were

I can’t say, though, that I’ve ever heard of any significant amounts
of diamonds found in anything more advanced in the process than
ordinary sediments. That means muds, silt layers, sandy ocean bottoms
near the outlets of waterways that had washed down the dimaonds, etc.
There are quite a few major diamond deposits that are indeed,
alluvial deposits of this sort. Because the action of eroding the
kimberlite and washing the diamonds downstream tends to fracture or
break up flawed crystals, such deposits often produce stones that
average higher quality than primary kimberlite deposits.

However, in wondering whether alluvial diamonds could end up in
metamorphic rocks, you do have some time constraints here. Most of
the major diamond bearing kimberlite pipes were volcanoes that
erupted during a somewhat defined geologic period some 60 to 100
million years ago, a time when the continent of Pangea was breaking
up, and the earth went through a period of greatly increased
volcanic activity over all the major land masses. (I may be
remembering some of this wrong. But it’s right at least in the
general drift of it, I’m pretty sure.) That means that there are
limits on just how much can happen after this. The resulting
volcanoes have to erode away, and still leave time for the alluvial
diamond deposts to form first sedimentary rocks, and then be buried
deep enough and involved in enough further geologic activity to
become metamorphic rocks. All this takes time. There is enough time
between now and when most of those Kimberlite pipes were formed for
this to have happened, if things went right along. Further, if we’re
talking about metamorphic rock, they don’t just form everywhere.
Metamorphic zones tend to occur in mountain building areas where
there are uplift pressures, like the lifting of the rockies or the
himalayas, etc, occur, or they occur when igneous formations, like
intrusions of lava into existing rock layers add heat and pressure.
Again, this doesn’t just happen randomly. And in order to not only
produce metamorphic rocks from sedimentary deposits that included
diamonds, if you’re going to then find the diamonds, those
metamorphic rocks have to then become exposed by additional geologic
activity and erosion or you’ll never know they’re there.

But that’s mostly conjecture as to whether it could happen. I took
geology courses back in college but I’m not, in the end, a geologist.
So there are certainly things I don’t know or are remembering wrong
here. But as I said, I’m not aware of any significant diamond
deposits in metamorphic rock, or for that matter, even in sedimentary
rock. Just primary sediments themselves, and those are surface
layers. But I’ve been wrong before. Just because I’ve not heard of it
doesn’t mean it can’t exist…

However, metamorphic rocks do play an important role in the
formation of other gems. Corundum (sapphire and ruby) often are found
in what were once limestones that were close enough to igneous
intrusions to be heated up by the nearby molten lava. That
metamorphic zone is where they’re found. A number of types of garnet
typically form in metamorphic rocks. You won’dered whether diamonds
might be found in shale. I’ve never heard of that, but I’ve got some
nice dramatic examples of large garnet crystals imbedded in shales.
There, the heat and pressure of the metamorphic activity allows the
crystals of garnet to actually form and grow in the shale.

And similar igneous intrusions often lead not just to the primary
granite deposits from the main igneous intrusions, but as well, to
seams of granitic rock, called pegmatite dikes, that form in cracks
in the host rock into which those lava domes intrude. The intrusions
don’t reach the surface, so they cool slowly and crystalize rather
than forming lava deposits. The resulting coarse grained granitic
rocks were connected to, but cooler than the main intrusion, and
subject to concentration of those minerals that crystalize at lower
temperatures. In addition, there are often higher amounts of water in
these rocks during formation, which also transports and favors
concentrating and depositing the lower temperature minerals. The
result is that this type of formation is where many of the most
interesting mineral resources and gem resources are found. Gold and
silver are often found in such rock, as are things like quartz in
all it’s gem forms, tourmalines, beryls, and many others.

Peter Rowe

Diamonds are found in kimberlite, 

found “only” in kimberlite? Where else?

What does it take to make the strongest C-material known to man?


What is powdered up on Moon regolith? Any feldspar?

If so, would it get consolidated over time and become shale? Might
it carry diamonds?

Might it then be subjected to high temp and pressure to form

There is no explosion of ejecta in kimberlite when diamonds are made
in the lab.


If the kimberlite dogma reigns supreme, where else will us
prospectors search for diamonds? To look elsewhere is heresy.

“All this takes time”

How much time to make lab diamonds, emeralds, rubies and sapphires?

found "only" in kimberlite? Where else? 

Brought to the surface only in Kimberlite lava’s. But thereafter, can
be found wherever they wander after being brought to the surface and
eroded out of the Kimberlites. Water is usually the eroding agent,
and alluvial diamonds are found in waterways, ocean shores off those
waterways, or the like. There are also occasional diamonds found
throughout the midwest of the U.S. and Canada, where glaciers
deposited them after scouring them up from Kimberlites further north.
Those are unpredictable as to location, but a few nice ones have been
found. And nano sized diamonds can be found in may residue layers
left after meteoritic impacts, both from diamonds actually formed by
the shock of those impacts themselves, or simply carried to earth by
the meteors themselves, some of which have similar tiny diamonds.
Again, these are microscopic, of value to science and geology, but
not so much gemology.

What is powdered up on Moon regolith? Any feldspar? 

Don’t know. Never been there. Are you planning to visit soon?

If so, would it get consolidated over time and become shale? Might
it carry diamonds? 

How would it become consolidated do you think? There are no plate
tectonics or similar active geologic processes going on there now,
other than the occasional impact from meteorites and the like. The
moon does not have a liquid or plastic or otherwise motile interior,
and it’s not accumulating lots of sediment either.

Whether it carries diamonds is another question. Some meteorites do
have diamonds, normally very tiny ones. And impacts with the moon
might, if carbon or graphite is present, make more of the nano sized
ones. But I also doubt that those diamonds would have any importance
to anything in the future other than the academic interest of knowing
they’re there

Might it then be subjected to high temp and pressure to form

The only source of high temp and pressure on the moon are the
occasional very brief occurances of such when a meteorite impacts.
And that can, with the right stuff present, form nano sized diamonds.
Such diamonds are also commonly found on earth in the layers of
dust/residue resulting from terrestrial meteoritic impacts. But the
sizes of these things are too small to be of any use other than
perhaps, if you took the time to collect enough of them, an
expensive source of diamond polishing powders… Cheaper to make em
in the lab, though, than try to collect the stuff.

There is no explosion of ejecta in kimberlite when diamonds are
made in the lab. 

What are you talking about? When diamonds are made in the lab,
Kimberlite has nothing to do with it at all. Kimberlite is a type of
lava, that’s all. It’s characteristic of a certain type of volcano
known both for the extreme depth from which the eruption originates
(around a hundred miles down within the upper mantle), and the speed
and violence with which these volcanos erupt. That allows the
kimberlite lavas to carry with them, on the way up, other minerals
that may be present as well in those mantle layers, including
diamond crystals. But the Kimberlite is merely the conveyor belt.
It’s not critical to the actual growth of the diamond crystals, and
when diamonds are made in the lab, you’re quite correct. There is no
explosion of Kimberlite magically appearing in the area. It would
take magic, since there wasn’t any Kimberlite or anything like it in
the lab to begin with.

Peter Rowe

found "only" in kimberlite? Where else? 

There are also aluvial diamonds found in panning the rivers in South
America and other places. But these diamonds started out originally
in Kimberlite and through some sort of geologic event or series of
events got washed down stream in a river.

Rick Copeland
Silversmith and Lapidary Artisan


People bring me those diamonds all the time to reset.

The conversation usually starts off with," this was my grandmother’s
engagement ring, I know it doesn’t look like much but it has great
sedimentary value"

If the kimberlite dogma reigns supreme, where else will us
prospectors search for diamonds? To look elsewhere is heresy. 

Not heresy at all. Look wherever you wish. Search parking lots,
mountain tops, or the bloody moon if you like. Nobody will get in
your way. However, a study of diamond formation, the conditions
needed, may convince you that the means to grow diamond crystals in
nature are not at the surface, and they need to be brought up from
well below the earths crust. Kimberlites are the best example of
such a means. Kimberlite pipes are not found randomly in any old
patch of ground, and ways for diamonds to leave those original
surface locations similarly are not unlimited. But if you feel like
trying something else, go for it. If you feel you’ll have better luck
with certain limestones, well, be our guest and good luck.

Frankly, I would suggest jewelry stores. Much easier, and does not
require a knowlege of geology.

Most successful prospectors are that way through careful study of
geology in principal, and applied as found in various regions,
looking for telltale signs, or data that should be suggestive of the
possible existance, as yet unknown, of certain specific types of
formations. That takes considerable research and knowledge beyond
asking here in Orchid. If the answers you’re looking for were as
simple as you seem to hope, we’d all have already staked our claims.
And might I remind you that Orchid is mostly a discussion among
people with knowledge of jewelry, and gems as they apply to making or
dealing with jewelry. Few on the list have a background in
prospecting and geology. Busy and popular as Orchid is, you might
find that there are better places to look for accurate detailed
geological University geology departments or university
libraries or government mining bureaus or the like, come to mind…

How much time to make lab diamonds, emeralds, rubies and

Seconds to months or years, depending on the gem, and the method. Too
broad a question for a meaningful answer.

Peter Rowe


Again, the diamond pipe in Arkansas and the Argyle pipe in Australia
are not kimberlites, but lamproites. Others exist as well.

Wayne Emery
The Gemcutter


Diamonds are also found in eclogites, which are peridotite and
garnet. These are common sources of diamond in Russia and the garnets
have a particular composition which makes them especially useful as
an indicator of diamondiferous deposits. You also get diamond
eclogites in southern Africa, Botswana and Angola being places that
have kimberlites and eclogites. the source of these rocks is very
deep in the earth’s crust to possibly the upper mantle. very high
pressure and temp, at least one location is the result of a meteorite
impact punching a hole through the earth’s crust eons ago when it was
thinner. Makes you wonder how much has been added to the earth by

Nick royall

People bring me those diamonds all the time to reset. The
conversation usually starts off with," this was my grandmother's
engagement ring, I know it doesn't look like much but it has great
sedimentary value" 

Good one. Thanks for the chuckle.

Mother Nature is the best jeweller there is and if you consider this
one universe with its mora than billions x billions of hot rocks and
cold rocks in the heavens, the Queen of the Heavens has lots to work
with and lots of possibilities for her jewelry.

How far can she go with nanodiamonds in water and sedimentary
action? When I was a child I found two species of shell fish fossils
in sandstones of the Ottawa Valley. They were replacement fossils
with perfect, well-defined quartz crystals. How long does it take for
her to use the water and sediments to make such gems? Ever try to
dissolve glass?

Dogma without reasoning does not serve science well. A few years
back I was reading up to get ready for a new round of prospecting in
my retirement. I read an article by a geologist who I take to have
been a plutonist. But he defended his colleague, a neptunist who
thought that granites may have been the result of sedimentary action.
When you have billions of years to develop that sedimentay value,
what are the limits?

When it comes to the particulars of nano-C, does anyone here say
with dogmatic certainty that diamond dust on a nanoscale could not
crystallize in cold water given billions of years of time? Could
kimberlites even serve as filters which trap fine loose sediments
including nano-C and turn it into diamond? Just because you can make
diamonds, rubies, emeralds and sapphires in the lab using magma baths
with high temperature and pressure does not mean they are always
formed this way. Maybe both plutonic and neptunist forces can be used
to crystallize and grow diamonds.

Speaking of growing, most of my prospecting now is in agrogeology. I
also do backyard tests of mineral dust. Microbes are the best miners
and when I briefly studied geology at Carleton U in 1962, nobody
taught about deep rock microbes and catclysmic forces (like big
rocks from the sky) were downplayed. We learn. What is marcasite?
Could microbes in peat, clay, coal etc. grow marcasite crystals? I
had this funny thought. What if microbe miners could use Be and Al in
their metabolism? What kind of minerals would they produce?