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Gem purity


#1

Though gemology experts like GIA will say clearly and almost
infallibly if X is a diamond, ruby, sapphire, emerald etc they do
not say to what extent the crystal mass is a pure mineral. My guess
is that there is a huge range in crystal purity. Let me explain why I
present this question. Recently I decided to send in prospecting
samples of magnetitite crystals from two different sites. There is
no mistaking magnetitite at the bottom of the gold pan. The samples
under magnification still appear as homogeneous. The two samples
cannot be distinguished visually. But one came back from Bureau
Veritas as 30% Fe and the other 23% Fe. In addition, the profile of
53 elements differs greatly. Orchidians can also do the numbers on
the Fe3O4 formula of magnetite. Fe has an atomic weight of 56 and 56
x 3 = 168. O has an atomic weight of 16 and 16 x 4 = 64. Even if all
the Fe is bound in magnetitite the Fe3O4 in either sample is still
less than 50%. That means greater than 50% of what appears to be
pure magnetitite is not magnetite.

What % of gems, though identified correctly by GIA and other
experts, are PURE? At an early stage in the mining-distribution
process where it is economically affordable, should a batch rating of
purity be generated?


#2

Gems are, generally, not pure! Just think of garnets, for example.
How would you define purity for this huge and diverse variety?

How would you differentiate between emeralds, aquamarines and other
types of beryl? Colorless beryl is probably “purer” than any other
type but worth much less.

I think your confusion is due to trying to compare ore samples with
gemstones.


#3

Dear Peter,

You are applying the process of assaying rock/mineral samples for
ore production to A simple example, ruby is yhe mineral
corundum, which is aluminum oxide. If the gemstone was pure then it
would not be red. Chemical impurity is what gives the ruby its color.
Likewise most colored stones. Gemology does provide a measure of
clarity, not purity.

When I first started doing gem identification, I had to learn a
whole different process from that I had learned in mineralogy and
petrology classes. Although composed of minerals, gemstones are
valued and identified by different methods.

I hope this provides some insight into the difference between the
disciplines. As a geologist, I had to change my thought and testing
process to do

John


#4
Gems are, generally, not pure! Just think of garnets, for example.
How would you define purity for this huge and diverse variety? How
would you differentiate between emeralds, aquamarines and other
types of beryl? Colorless beryl is probably "purer" than any other
type but worth much less. 

(clip)

Thank you, and JR for the interesting feedback. As JR says, gemology
is not geology. I am a layman in both fields but I like to get a
good deal when I buy any kind of stone work whether it is a stone
wall for my garden or a ruby. There are many tests and measures
beyond the naked eye of a layman which determine market value in
either case. Perhaps “chemically perfect” would be better wording
than “pure”. As JR says, the redness of ruby is an impurity which
causes the colour. We know the chemical formula for pure
corundum-alumina (Al2O3). What is the element profile of the red
impurity in ruby alumina or the blue impurity in sapphire? It is
surely of academic interest. Is it of market interest? Could it be
assayed? Yes, as long as the mine is producing similar gems in
quantity. A one gram representative sample can be assayed and that is
a very small cost. Beyond academic and market interest in this issue,
surely those who make diamonds, rubies, sapphires and emeralds in the
lab want to know all about the chemical perfecting of colour. How
would we differentiate the different types of beryl? By assaying for
the chemical colourants. In “A Golden Nature Guide” called “Rocks and
Minerals” I read, “Oriental emerald, a green corundum gem, is harder
(than emerald) and may be the most valuable gem” (page 84).


#5
In "A Golden Nature Guide"called"Rocks and Minerals" I read,
"Oriental emerald, a green corundum gem, is harder (than emerald)
and may be the most valuable gem" (page 84). 

A fine example of “don’t believe everything you read”. Most
commonly, the phrase these days applies to stuff found on the web,
but published books are not immune (nor are posts here on Orchid,
including this one, for whatever it’s worth).

“Oriental emerald” is not a generally accepted name for green
sapphire. As a general observation, when a gem of one variety (such
as green sapphire) gets labeled as something else but with a modifier
to differentiate it from the real thing, it’s usually safe to assume
that what it’s trying to pretend to associate itself with, is the
rarer or more valuable gem. Citrine Topaz isn’t Topaz. It’s Citrine
quartz. But sellers switch in the word topaz because that’s usually
the more valuable gem. Likewise, a fine emerald will almost always be
substantially more valuable than a similarly looking green sapphire,
though in truth, a green sapphire with the intensity of green of a
good emerald would be quite rare (I can’t recall ever seeing one, to
be honest). The green sapphires labeled oriental emerald that I’ve
seen generally have been fairly poor quality material, a long ways
from “the most valuable gem”.

As to differentiating gems and testing them, unlike general
mineralogy, most fine gems (at least the transparent types) tend to
be single crystals, or at most, single twinned crystals. In short,
materials upon which optical tests can be performed that do no damage
to the gem. These can identify the type of mineral in the vast
majority of cases, without needing to resort to the types of "assay"
methods used with ores, which often are at least slightly
destructive. In determining things like the nature and analysis of
the impurities that can give gems their color, it’s not generally
valued to know the exact percentages of, for example, the Chromium
that gives ruby it’s red color. Determining that it is indeed
Chromium that’s doing it, instead of something else, is usually
what’s important, along with any other impurities that may modify the
appearance (such as iron altering the color of ruby, usually for the
worse) and after that, what’s evaluated is the actual color, not
usually the precise chemistry of it’s cause.

The exception is when it’s desired to determine the actual origin of
the stone, which sometimes can be done not just down to a region
where a given stone is found, but in some cases, even the actual mine
(though that’s less common.) Here, useful tools will be examining and
determining not just the impurities that give a crystal/gem it’s
color or properties, but as well, the identities and nature of the
inclusions, bits of other minerals trapped in a gem, like little
garnet crystals sometimes seen in diamonds, for example. Also, when
the exact nature of the impurities is desired, tools like
spectroscopy are often used, because these, like other optical tests,
are not damaging to the gem. There are other tools more advanced labs
can use, such as electron microprobe, x-ray fluorescence, x-ray
crystallogrophy, and more, but some of these may require at least a
small sample that is damaged in the test. Normally, the size of the
needed sample is very very small. Nothing like your suggested one
gram sample.

Hope that helps.
Peter Rowe


#6

your main problem is using the wrong analytical method to determine
mineralogy. Essentially you are using a method that gives you a bulk
analysis of the elements present such as XRF when you should be
using a method that determines either spot analysis such a EPMA or
laser ablation where you can analyse a chosen point on a sample that
is as small as 20 microns or XRD to determine the crystal structure
which will then confirm magnetite.

Mounting your grains in resin and polishing will allow you to use a
reflected light microscope to determine what is what as well.

My suggestion is that if you are certain what you have found is
magnetite then you should not spend money telling you something that
is worthless as far as identifying a single phase goes.

As for your second question, it is like asking what part of a
Kimberlite pipe is diamond any how do people know? Well, they
extract the diamonds and use a microscope to determine the important
bits, which is what you should be doing.

Nick Royall