Saturday, October 11, 2008

Buying Turquoise, Part II

By Anita of Deerwoman Designs

Part II

This section will include information on testing, characteristics, composition and some of my personal experiences with turquoise.

There has always been much discussion about terms to describe stones and their treatment.
Much of it centers around what is "natural", "treated", "enhanced", or "stabilized".

Since turquoise is a soft and porous stone, that occurs as nodules, veins or a crust on matrix, it rarely occurs in large enough volume to produce a sizable piece of natural turquoise of uniform color. If it is a large stone it is unlikely to be natural or stabilized turquoise unless it is very expensive.

You can see irregular areas and nodules incorporated in the matrix in many of the illustrations I have provided. You can tell that they would not yield any sizable chunk of uniformly colored turquoise.

The basic chemical formula: CuAl6(PO4)4(OH)8·4(H2O)
The minerals: Aluminum, Copper, Phosphorus, Hydrogen, Oxygen:

Even more rare are turquoise crystals, so much so that you do not see these used in jewelry. For this and a beautiful picture of a turquoise crystal and its properties see:

http://www.webmineral.com/data/Turquoise.shtml

There are many variations on a theme in turquoise formulas, different minerals that contribute their own characteristics to the turquoise at hand. Expect to see a lot of differences in the formulae of tested specimens.

Turquoise occurs mainly in arid areas with copper deposits. Weathering may form a crust of turquoise, or it will cause filling in areas in rock, forming veins or nodules. The copper is what gives it the wonderful blue color. Iron contributes the green. Any real turquoise must have some copper, or it is NOT turquoise.

Knowing as little as I do about turquoise, sometimes the line between matrix and the turquoise itself becomes a little fuzzy:

For example:
On the right is a green turquoise nugget, but you can see a tiny area of pure sky blue turquoise.

On the left is a nugget with both the greenish and bluish turquoise existing side by side, transitioning from one to the other.


These examples show nodules in the matrix, webbing, and irregular large areas of turquoise interspersed with the matrix.


Natural turquoise will have had nothing done to it except cutting and polishing. As discussed in Part I, the most common treatment is stabilization, which involves strengthening the stone by adhesives or other binding materials. This is an addition to the natural components of the stone and not only changes the hardness, but also often the color.

A proprietary process called the Zachary process is also applied to turquoise. Although details of the process are not revealed, it is a patented technique involving adding chemicals, similar to the composition of the turquoise, heating it and running an electrical charge through it. This process can be detected by laboratory tests for certain earmarks, but it basically does not change the chemical structure or color of the turquoise. However, it hardens the stone and stabilizes the color, so that it will not turn greenish with wear and time.

For good examples of both natural and Zachery treated turquoise see
http://www.monsterslayer.com/Pages/Turquoise/Turquoise.aspx

Some say turquoise treated this way is still natural turquoise, others say it is treated or "enhanced". However, the process is much less invasive than stabilization or dying and preserves the look of the stone exactly like it was in its natural state.

Sleeping Beauty turquoise is stabilized with the typical stabilization process, but only on the request of specific buyers.

These interesting videos at the Sleeping Beauty website gives a good look at turquoise as it comes out of the ground and is sorted and graded.

http://www.sbturquoise.com/

Tests: There are a variety of tests that are used to determine the identity of a mineral or stone. These, also used to identify other types of gemstone, include hardness, streak test, magnetism, type of fracture, refractive index, specific gravity, and other more intensive tests.

These are done by professional labs, though you can do a rough at home hardness or streak test to approximate the hardness of the stone based on the Mohs Scale of hardness. The hardness test basically involves scratching the specimen by a harder substance for which the value of hardness is known. and using the specimen to scratch softer substances to determine its position on the scale. See http://www.galleries.com/minerals/hardness.htm

Minerals have different colors when you scrape a specimen agains a hard, rough ceramic surface. You can't do this with the harder minerals of course. You can find out the color of the streak of various minerals from mineral databases. Often, however, it does not reveal much. If the speciment streaks white, this is the color streak of a majority of minerals. It is wonderful though when you have a mineral with a colored streak and your specimen does have the right color! Turquoise should give a white or very light bluish green streak.

Turquoise's hardness will vary in its natural state, even from the same mine. Even with stabilization, the hardness also varies greatly. Typically, turquoise hardness is between 5.5-6.5.

http://www.jewelrysupplier.com/2_turquoise/turquoisel_properties.htm

Common sense tells you that if it melts, it's plastic. if it shatters like glass, then it most probably is. If it is very lightweight, it could not be stone.

If you need to know what your turquoise is, you will have to test it in some way. Since some home tests can be destructive, you may need to get it tested by a lab.

Here's one that puzzled me.

I obtained some dark blue green nuggets a number of years ago, varying in size. They have a very waxy feel, which I did not like. I suspected they had been dyed and treated with some kind of oil or fat. I was told they had been oiled.

They look like Tibetan turquoise, with nodules in a dark matrix. I had one broken sample. Its interior color is consistent all through the nugget, the same dark color as the outside.

I soaked some in water and dish detergent for 24 hours and scrubbed.

They turned out to be a light blue green turquoise color, without the waxy feel.


I tried to break a soaked one open. NOT so easy, I whacked it 20 times with a hammer, dented the work area and only managed to break off a chip. The inside was the same original dark color. This didn't make sense. So I asked help in breaking it open. This time I could see the dark interior, consistent with the interior of the unsoaked one. I found some very small bubbles!


This could be a kind of faience or glassy composite with a strong adhesive added to make it very resistant to breaking. Real turquoise would not survive such procedures. Faience can be produced with the color consistent throughout, instead of just in the glaze, but it would be much softer than this example. Originally I wondered if it was a chalk turquoise or another white mineral treated with a very strong adhesive and enough pressure to infuse the whole nugget with the dye. But it is strange the way the dye at the surface was lightened so much by the soaking. Also notice how the "matrix" is only at the surface in this example. And why the bubbles unless it is glass? Could it be plastic with a fake matrix at the surface?

Guess what, it melted and burned! If you have a piece that can be destroyed, try the hot pin test or burning first. it may save you a lot of time.

I was amused to find a photo of "gaspeite" in the mindat.org forum/rockhounds that look like my mystery ones, but they are green. The owner was asking if they are gaspeite, as they were claimed to be. The photo can be seen at http://www.mindat.org/index.php

From that site go to:
http://www.mindat.org/forum.php?file,11,file=6794,filename=T526.jpg.



Examples of "chalk turquoise":


The sample at the far right is a large broken chalk turquoise beads with a matte finish. The vendor said he had treated them to give them this matte finish. The surface of the originals was a little more glassy as you see in the nuggets to the left here. They have the appearance of chalk turquoise with the faint variations in color and minor amounts of matrix. When broken open the color of the bead at right is consistant throughout except for a couple small spots that did not take the dye or were a different mineral altogether. I am not sure what the beads really are, chalk turquoise or perhaps dyed magnesite or other white stone. They were very easy to break.

Unless you have your turquoise tested you'll probably never be sure exactly what you've got.
My experiences here reveal just how little I do know. It takes a long time and looking at a volume of different types of turquoise to learn anything at all.

The best rule of thumb is to buy from a reputable dealer. This is a must if you are making an investment in turquoise. A reputable vendor should provide verification of what you are buying.

Most important, if you like it and it fits your budget then you will never lose. There is nothing wrong with using stabilized, "chalk turquoise" or dyed magnesite. They are very beautiful, durable and affordable materials.

You just should know what it is you have, and you should be told at the point of purchase. And when you use it in jewelry to be resold, please label it correctly.


All rights reserved

Additional turquoise references:

Look under T at
http://www.rings-things.com/gemstone/index.html

http://www.mindat.org/min-4060.html
http://www.bwsmigel.info/GEOL.115.ESSAYS/Gemology.Turquoise.html
http://minerals.usgs.gov/minerals/pubs/commodity/gemstones/sp14-95/turquoise.html
http://www.americana.net/jewelry_turquoise_origin_article.html
http://www.collectorsguide.com/fa/fa098.shtml
http://nevadagem.com/pages/turquoise.html
http://en.wikipedia.org/wiki/Turquoise

Definition of Zachary process and others:
http://www.turquoiserough.com/treatments.shtml

See the post by
JB posted on Thu, Feb 14, 2008 at the following website:


http://www.gemologyonline.com/Forum/phpBB2/viewtopic.php?t=4859&view=previous&sid=e27049cec624b1ffa8d4fc0510a5c4ac

Historical turquoise: http://www.metmuseum.org/toah/ho/07/wai/ho_1989.87a-l.htm

1 comment:

Dave Robertson said...

Hi, Deerwoman,

Another illuminating post to help people understand the complicated world of turquoise! (Applause.) Very nicely done.

And thanks for pointing people to our Gemstone Beads Index "turquoise" entry. Much appreciated!

--Dave
at Rings & Things

Copyright

All rights reserved for all content and images in my blog by Deerwoman Designs (TM)