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.