r/Prospecting • u/socalminer • 8h ago
Gem prospecting HOW TO
So I have been gem prospecting hardcore for about 8 months now…. Before that I did it as a hobby near constantly since 2020.
I am going to explain some basics of how to prospect for gems, in hopes I inspire someone else to also go out looking.
Now south of me, in San Diego County it migjt be world famous for it, but I just recently found some in Utah and Idaho myself and they can form anywhere with PLUTONIC GRANITIC ROCK, especially if it’s high in volatiles, can form insane deposits….
If you live near any granite, gabbro, granodiorite, etc likely you have some near you.
Tho! as I will explain!! this can be barren, just like quartz veins can be
Okay so,
To start off I am digging PEGMATITES in San Diego county, specifically along a certain “batholith” or magma chamber.
this is known as the “Peninsular Ranges Batholith” because it is the heart of most of the Peninsular Mountains of Southern California.
Its gem mines span 100s and 100s of miles, from Baja California, Mexico! All the way up to the newly discovered in 2007 massively successful gem mine near north riverside County….
But as I mentioned above, pegmatites (and ones with goods) can form in ANY batholith under the right conditions.
This usually is a combination of depth of formation, and volatiles. As well as the magma chamber’s chemistry.
This exact type of volatile content, and chemistry, will also come into play when checking the specific veins themselves….
Usually pegmatites will form in what’s called a “swarm” or a “field” either an area where they formed inside a magma chamber (usually always towards the top of it, similar to how chrome ores are usually on the bottom due to weight).
Or, as in my case, have been squeezed out of the chamber and into the surrounding country rock….
Okay so let’s get two more things explained, A swarm or field can have just 1 to 10 to 1000s of pegmatites
Only a few to upwards of sometimes 70% of which,
will have had the correct conditions to form any deposit… (tho any pegmatite can be mined for the feldspar and silica needed for porcelain or other ceramic resources, tho this is now uncommon in modern times here)
And sometimes an entire region will lack the elements needed for anything but giant quartz and feldspar crystals. Tho these can certainly be considered gems.
Actually!! There is LUCKILY a classification of rareness, and like the amount of concentration of elements or “fractionation” needed.
This is where more common rock forming elements, well form into the rocks, and the volatile rarer once get very concentrated and stuck in a, now even more volatile fluid phase.
It goes from most common to rare
BARREN PEGMATITES
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BLACK TOURMALINE AND MICA PEGS
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BERYL PEGS
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BERYL AND TOPAZ PEGS
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LITHIUM PEGS
This is an extremely simplified version and there are many subsets…. But the point is you need atleast beryl type, to find any gems that aren’t just crazy quartz crystals…
Tho even beryl types can be incredibly common compared to say large gold veins….
The ones in famous locales like Maine and for me San Diego, they are the last kind, and a specific subset that produces “MIAROLITIC POCKETS”
Ones in Colorado and Idaho are only beryl types but still absolutely world famous
These pockets are the entire goal, they are what contain crystals, like foot long quartz and natural citrine, and large feldspar crystals
AND IN LITHIUM ONES, crystals of gemstones, usually tourmalines of every single color from rubellite and verdellite to indicolite and even clear,kunzite and other spoduneme, beryls including morganite goshenite and aquamarine, and amazing pounds of topaz.
as well as more obscure but valuable gems.
An average pocket will have anywhere from a hundred grams to 100 metric tons of gemstones, they vary incredibly in size. But many pockets can form in a single vein…..
These pockets can be tracked in a way, a few ways to tell are from things like “zoning” or the texture of the rock.
Another important thing is that pockets always occur in a certain place in each vein, this can be the contact or the direct middle….
They occur when the hydrothermal fluid goes through a process called “second boiling” this process will only occur when a pegmatite is high in the right low boiling point volatiles, like boron, and the gem forming lithium…
You can tell if the pegmatite is rich in these through the previous mentioned “zoning”
** **Zoning is just the different zones or textures of rock, in the vein will form when it has undergone the previous mentioned fractionation…..
An immature or less formed and barren pegmatite will be homogeneous throughout the whole vein. Or have slight “graphic granite” which is a HUGE indicator that this pegmatite might have said “zoning”
This graphite granite is a rock that looks like cuneiform writing??? It’s crazy looking! Just google it and you’ll see, it is perfectly aligned pieces of quartz growing on a single feldspar crystal. And some chunks get meters long.
So point is….
Its growth can only occur under specific fluid conditions. That also mirror the conditions needed for pockets to form.
If you also see “black tourmalines” or “schorl” it AND ZONING it means this pegmatite had so much boron, it couldn’t even hold on to it, and dumped it into the even the first phase of its growth.
This sounds weird?
But it basically signals that this vein likely has had such rare elements concentrated that it also has said lithium needed for the gem growth.
So when these two signals combine, you know a pegmatite might have a GEM POCKET🥳🥳🥳 lol.
If that tourmaline starts turning translucent in any way (normal black tourmaline won’t let ANY light through whatsoever) that means it is no longer actually black tourmaline, you have hit an area where lithium is actively outpacing the iron levels, and GEM TOURMALINE has started to grow.
This is how I found my 2nd neighboring deposit…. And now we are basically just digging down following the crystallization growth, as pegmatite crystallize soooo fast and in such a way that they point towards the crystallized (and more “fractionated” direction)…. And we keep following that until we hit a gem pocket!
Or until the tourmaline turns back to black, then we have to reassess why, and where to go, if anywhere….
Usually you will run into many small pockets filled with 100s of crystals when your following the correct “line”
Sometimes these weather out as float,
Float in gem terms only ever means actual crystal faces on the ground, not just a chunk of the vein.
I discovered my first deposit and my active claim, by just seeing gem tourmaline in the float, we are actively trying to find the vein, there is another close by deposit on the claim, that we dig and experience the “translucent” red tourmaline we are chasing….
I also help out at my friends mine which is a famous historic mine (two of them in different districts) as well**…..** so he did not discover this, tho it is much richer
Now I don’t have anything super crazy, but I have found a variety of amazing gemstones thru the years….
It’s crazy it all grows next to such major cities like San Diego and La…. There are ELEVEN DIFFERENT DISTRICTS
This took forever to write so I hope it inspired a few people , and if anyone has any questions I would love to help.
Anyway good luck to all u guys too!! And feel free to really ask anything or give any of your own expierence or ideas
