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Pure mineral-gems are usually monochromatic


#1

True or False?

Is it not mineral impurities which almost always cause the colour
variations in mineral-gems (vs rock-gems)?

How many colours does pure SiO2 have?


#2

Purity is one of those funny words. What one calls impurity another
calls character. Corundum (aluminum oxide) is clear. Add some
chromium into the mix while the crystal is growing and you’ve got a
ruby. Is the chromium an impurity or simply an addition? In the
strictly technical sense the coloring elements are impurities, but
one dislikes the generally derogatory connotations of the word.

Anyway, if you’re interested in the insides of gems, the book you
want is Edouard Gubelin’s “The Internal World of Gemstones.” It’s
out of print, but used copies can be found.


#3

Not always true, you get colour variations due to radiation (ie
smokey quartz and topaz)and occasionally due to different light
vibration properties of different crystallographic axes of a mineral
(pleochrosim, as seen in iolite, biotite and others). There are other
classes of minerals that have what are called end members that make
them coloured such as apatite, tourmaline, where fluorine, chlorine
or hydroxyl ions give a different colouration. Garnets are coloured
by the structural cations and impurities give rise to colur
variation, not colour itself.

You could say that it is the impurities in the big 3 gems that
almost always make them without the small amounts of Cr,
there would be no ruby or emerald.

Nick Royall


#4

Not being a gemologist I’d rather not opine. I suspect that the GIA
and other trade organizations have done, or have access to, all the
sort of quantitative analyses in which you are interested. Given your
interest in these technical details I suspect you’d enjoy pursuing a
gemological certificate, or at least undertaking an extensive course
of technical reading on the subject.

Elliot


#5

Yes, words are very important in fashion and artistic fields.
According to this Audubon guide Al2O3 can be just about any colour. I
assume that a related mineral-gem like ruby or sapphire would be >90%
pure Al2O3. But how often is a gram of ruby or sapphire ever
sacrificed to do an assay of elements? Has anybody done it with
diamonds or any others in the super-expensive category of gems? It
could be done affordably with the grit residue of these industries I
suppose. The commercial implications are probably minimal but is
scientifically interesting.

OTOH, when you see a gemstone carving of Haida argillite or jade, I
expect that the assay of elements is going to show much more of a
mixture of minerals. I would not expect argillite to be even close
to pure feldspar or nephrite jade to be even close to pure
actinolite-tremolite. Do you agree?


#6

Ruby and sapphire are more than 98% pure Al2O3. To analyse them you
dont need to sacrifice any material, the testing is all
non-destructive, Rahman spectroscopy, fluorimetry, x-ray analysis of
one sort or another. You can use laser ablation or ICP-AES which
will leave a 5-20 micron crater in your sample but you only need to
crush up material for whole rock analysis when you want to do whole
rock analysis.

elemental analysis of Argillite is almost pointless as it is a
metasediment and can therefore contain almost anything (BUT WILL
HAVE A TYPICAL ANALYSIS WHICH DOES NOT CHARACTERISE IT UNIQUELY AS AN
ARGILLITE). Spending money on a whole rock analysis would be a waste
of money and material, a decent magnifying glass and some knowledge
of petrology is what you need. Pure actinolite/tremolite would be
classified as amphibolite, of which you can have nephrite as a
subclassification.

Yes, I have analysed diamonds an rubies by more than one method for
some interesting reasons but none of them to do with classifying
them as

Nick Royall


#7

Correct me if this summary of Nick’s posting is in error. Colour
variation in mineral-gems may be due to

(1) the geometry of crystallographic axes (perhaps C variations
would be an example); (2) chemical variations due to chemicals other
than those which define the mineral.

Under chemical variations there (a) elements and compounds (like Cr
in ruby or emerald) which impart colour and (b) ionic variations.

Variation in end-members IMO define a new mineral in a grouping as
in the feldspar grouping.

A standard field guide like the Audubon guide which I have here
glosses over the reasons for colour variations of most minerals
described but they are important as Nick illustrates in referring to
the cause of ruby and emerald colouration.

When the gem stone is a rock rather than mineral as is typical in
nephrite jade, Audubon gives a picture of nice, dark green jade rock
and then the associated text describes the mineral
actinolite-tremolite. But the colour variation may be even more
complex and varied because there could be scores of minerals in the
stone which give colour and pattern and Audubon is silent on this
issue.

What colour is pure actinolite-tremolite? Not rhetorical - does
anyone know? Has anyone seen pure or close to pure
actinolite-tremolite, eg in Smithsonian? I would guess at light green
or maybe even clear. But nephrite jade as a rock can be almost any
colour. In addition there are patterns as you can find in “marbled
jade” when included carbonates (of Ca and perhaps Mg) give a
"swirled" pattern.

As Elliot advised I had already sent samples to GIA and they are
good on identifying minerals but for more complex rocks like jade
rock, they advise petrographic analysis. I found the petrographic
analyses combined with assays of elements to be most helpful. Making
the calculations from Jade West nephrite assays of elements proved
that it was far less than 10% pure actinolite-tremolite.

What then determines the resulting colour of the final carved jade
other than the polish or light reflection in thin carved sections?
IMO it is the mixture of the many other minerals for the most part
and not the actinolite-tremolite. The actinolite-tremolite probably
imparts strength (toughness) to the rock as rebar imparts strength to
mortar or alpha pins to fire brick even though the rebar/pins are a
small part of the mass. You can see the fibrous minerals, sometimes
matted, in the petrographic photos.

Thus when it comes to the three gemological criteria of aesthetics,
rarity and durability the durability of nephrite jade may be
determined mainly by the “rebar effect” of actinolite-tremolite and
the colour may be determined mainly by scores of minerals other than
actinolite-tremolite. The value added to aesthetics by a world-class
carver may multiply the raw stone value by hundreds when an artistic
rarity is created. How much is a Bill Reid (Haida - Queen Carlotte
Islands First Nation) carving in humble argillaceous stone worth
after it becomes a totem figurine or a finely carved jewelry box lid?
The same could be done here in nephrite. A Sto:lo (Fraser Valley
First Nation) ethnogeologist believes the Sto:lo worked both nephrite
and argillaceous stone in centuries past. It became a lost craft in
their culture.

Earlier an interest was expressed on Orchid re the Yang book with
its taxonomy of 44 jades. I located the book through a Chinese
translator and will post my findings when I get this file reader
working. Leaming seems to dismiss Yang since he says almost anything
can be called jade in a Chinese carving factory. But this may be a
Chinese-English translation problem. IOW Yang may be pointing to a
taxonomy of genuine actinolite-tremolite or jadeite jade or maybe
Leaming is correct and Yang may be saying any pretty and carvable
stone is jade.


#8
When the gem stone is a rock rather than mineral as is typical in
nephrite jade 

Peter, I’m curious. Just what is your definition of a “rock” as
opposed to a “mineral”? So far as I know, a “rock” refers to a
material that is a mix of several minerals rather than a single
mineral (such as granite, a mix of distinct crystals of quartz,
several feldspars, micas, and sometimes other stuff too). But
nephrite jade need not be more than nephrite jade, which can be a
single type of mineral, rather than a mix of several. Same with
Jadeite. If the jades are “rocks” in this view, then that would mean
they are only partially nephrite, mixed with other minerals, rather
than being pure nephrite. Note that a mineral or specifical gem,
etc, need not be absolutely pure, as this thread has discussed.
Impurities mixed into a single mineral do not change the
identification of that mineral, nor does a variable composition of a
mineral when that mineral can have a varied composition, such as the
various series of garnet. While most garnets occur with a specific
formula for an individual crystal, there’s nothing in the rules that
says the crystal cannot vary some from one part of a crystal to
another, so long as it’s all along the same solution series for that
material… The nephrite I’ve seen, while it can have a jumbled
appearance, seems generally to me to be a single basic mineral with
a good deal of variation in impurities giving the textures and
patterns, rather than a specific definable mix of more than one type
of mineral…

Peter Rowe


#9
So far as I know, a "rock" refers to a material that is a mix of
several minerals rather than a single mineral (such as granite, a
mix of distinct crystals of quartz, several feldspars, micas, and
sometimes other stuff too). But nephrite jade need not be more than
nephrite jade, which can be a single type of mineral, rather than a
mix of several. 

Let me cut to the chase and put a theory (right or wrong) out for
discussion:

Nephrite jade is usually given its toughness/strength from a
rebar/rewire effect and its colour from minerals other than
actinolite-tremolite.

That is because what stone cutters and carvers like those who shape
the Jade West material here are working with is a rock (usual
definition) which carries far less than 10% actinolite-tremolite. I
would like to see a nice ring or set of beads made from almost 100%
pure actinolite-tremolite but I don’t know of any. Does anyone else?
Nick pointed out that precious stones can be “assayed” by methods
which do not destroy the stone so you can get corundum precious
stones with maybe 97% aluminum oxide. What if a 97% pure nephrite
jade ring or bead were compared to a 97% pure jadeite ring? What
would the jewellers say about “the best of the best” in comparing
the two? But beyond that, the best of the best may come from
admixtures of other minerals in the rock. That may be a chapter in
futurejade.com although I want to read Yang’s taxonomy of "44 jades"
first.

What does it say about rock-gem taxonomy for gemological purposes?
Future lab-made rocks may be given “grouping” designations like “jade
grouping” or “argillite grouping”. Is there any stone which will not
be lab-made in the near future? If not, one would expect lab-made
future jades and argillites to rate even higher on durability and
beauty than present natural stones. Rarity will depend on how closely
the “trade secrets” of stone making are kept.