Guest Article: Use of XRF in Identification of Turquoise Treatment. Neil Ray

The Use of XRF in Analyzing for Turquoise Treatment

Neil Ray

(Neil Ray is a geologist who has been active in developing X Ray Florescence XRF as a practical means to identify turquoise.)

A considerable amount of turquoise is available on the market, ranging from low grade to gem grade material, providing a wide range in budget for almost any consumer’s needs.  Additionally, if someone is looking for hard durable turquoise that mimics the look of gem grade without breaking the bank then an endless possibility exists of both stabilized and enhanced turquoise.  I would like to point out that there is nothing wrong with stabilized or enhanced turquoise, as it has a large customer base in the jewelry trade and many high-end jewelry pieces are produced with both.  However, the consumer should be aware of the stone they are buying, assuring that they are getting what they paid for.  Like provenance, buy from the miner or from someone who bought from the miner or purchase from a well-known reputable dealer that got it from the miner.  Unfortunately, in today’s world some unscrupulous dealers will pass stabilized or enhanced material as natural.  Just like provenance greatly determines the price of turquoise, treatment does as well, perhaps even more so.  For example, there is a great amount of stabilized Bisbee on the market that can be had for the cost of some mid grade turquoise at only $3/ct.  Some of this stabilized Bisbee would visually pass as gem grade natural that would sell for $50/ct or more. 

The focus of this discussion is to describe the varieties of stabilized and enhanced turquoise that exist in the market and how the XRF testing method can be used to not only detect the alteration, but to identify the type as well.  As most people are aware turquoise is too soft and porous for everyday use in jewelry, and it requires a hardening agent before it is suitable for use.  Turquoise is also highly porous, and it will readily absorb oils and contaminants, sometimes losing its blue coloration over time and turning green with prolonged exposure to the environment.  This disadvantageous porous nature allows for the added benefit for it to be sealed with an epoxy/resin or an applied chemical treatment to enhance the color.  It should be noted that some material is naturally hard with low porosity already, but it may lack the color qualifications to make it appealing as gem grade.  As such some material is treated for hardness only and some for color only and some require both.  I would like to mention that there is a great deal of confusion between the terms stabilized and enhanced, stabilized implies that the turquoise has been injected with an epoxy or resin, where enhanced is a chemical treatment that is less invasive and can either enhance the color or hardness.  The term treated is loosely used in the gem trade when it applies to turquoise.  However, enhanced turquoise certainly carries more value than its stabilized counterpart.  In addition to these treatments sometimes we encounter material that is not even turquoise at all but made to look like and pass as turquoise.  The most common of which is a dye used to color the mineral magnesite.  This dye rarely penetrates more than a few millimeters in the surface and if one was to cut or break the specimen, they would notice that it is completely white on the inside.  The material in question can also be soaked in a solvent such as acetone to reveal the hidden magnesite within.  Fortunately, some of this faux turquoise material passed as turquoise is obvious to identify and some not so much.  I would like to discuss some of the treatments and material passed off as turquoise and how the XRF method is utilized to determine them.  I have five specimens below that are either treated or material that is not turquoise.  Can you identify which is which? 

#1

#2

#3

#4

#5

Below is the complete XRF analysis that was performed on all five stones

Wt. %	1	2	3	4	5
SiO2	7.21	0.29	5.78	52.90	0.07
Al2O3	20.34	25.80	38.14	2.43	0.06
MgO	1.68	1.79	1.50	21.11	0.24
CaO	0.05	0.40	0.43	0.78	0.13
K2O	0.16	4.83	0.15	0.10	0.02
Na2O	0.31	0.46	0.86	0.05	0.33
P2O5	27.47	30.30	26.58	0.07	0.01
TiO2	0.18	0.03	0.01	2.91	1.52
MnO	0.01	0.01	0.01	0.16	0.00
FeO	0.15	1.16	0.35	0.03	0.00
Fe2O3	1.31	0.41	2.91	0.07	0.00
S	0.49	0.10	0.24	0.02	0.14
Cl	0.09	0.12	0.08	0.11	0.38
Trace Elements (ppm wt.)
Strontium	24	54	45	136	2
Barium	441	717	2187	36718	13
Rubidium	2	2	4	6	1
Zirconium	321	3	3	18	4
Molybdenum	2	15	35	10	8
Vanadium	158	30	143	10038	48
Nickel	35	26	79	672	8
Copper	71361	63409	66346	86	42
Zinc	1721	1768	1731	65	977
Chromium	209	82	280	53	21
Lead	5	5	6	14	2
Arsenic	13	6	196	16	2
Tungsten	71	284	325	76	42
Antimony	15	16	32	19	3
Tin	22	19	16	29	2
Silver	86	10	15	35	2
Bismuth	7	5	5	14	2
Niobium	2	2	3	6	6
Uranium	5	6	26	28	2
Thorium	9	2	2	17	1
Lithium	10.04	0.00	0.10	0.00	0.00
Total Wt. % plus Copper	66.57	72.04	83.70	80.75	2.91

The first stone was from one of my colleagues who had it in his collection for years and asked me to verify its origin and grade.  The stone is Sleeping Beauty, it has a nice eye appeal to it with an excellent polish and coloration.  The XRF doesn’t account for oxygen, which is the chemical balance of most rocks and materials.  To accommodate this difference, it is conventional to convert elemental percents to oxide percents using the formula weights.  Ideally an analysis should total 100%, but turquoise does contain water and so a 75% or greater is typically expected for most stones.  We can see on #1 that the total is only 66.57%, which indicates that we are missing a major component that is not oxygen or water.  That component is indeed epoxy or resin and the balance indicates stabilization.  Since epoxy or resin is a polymer composed of carbon the XRF can not detect carbon, and it will show as a low total balance discrepancy.  The second stone, also Sleeping Beauty, is a heart cab that a friend of mine gave me.  Notice that it also shows great eye appeal and a nice polish as stone #1 does.  If you look at the balance it is a bit low and coupled with other element parameters may indicate stabilization, but the greatest thing to notice on this specimen is the potassium, notice that K2O is 4.83%.  Most turquoise has low amounts of potassium and K2O will typically be considerably less than 1% in natural turquoise, unless it contains natural potassium bearing minerals such as feldspar and some clays.  The Zachery treatment is a proprietary process that uses a potassium compound and an electric current to both enhance the color and the hardness of the stone.  Zachery treatment is very difficult to detect and most certainly requires a chemical analysis for verification, as it seems the only apparent difference is the amount of potassium.  Additionally, some Zachery treated stones may show less potassium than others, which is why I also utilize trace element patterns and ternary diagrams to make the final distinction from natural.  Stone #3 is a bit harder to identify as problematic, it is a cab that I cut from Cloud Mountain rough that I purchased online.  Again, a nice-looking stone with decent spider webbing.  The oxide totals appear normal, and the potassium is low, so what is the issue?  Well, there is another less known method of enhancement that the Chinese began using, which involves soaking the material in sodium silicate to increase hardness.  Since sodium silicate contains silicon, it almost acts as an artificial quartz to fill in the pore space and harden the matrix.  This type of treatment is not well known, and it goes by the name of porcelain treated, stones take on a waxy appearance with a slightly lower density than their natural counterpart.  Unfortunately, the identification of porcelain treated turquoise is very difficult, it is obvious that treated stones will show more silicon, but silicon is also naturally occurring in quartz and a variety of minerals that make up the matrix.  However, porcelain treated silicon typically shows higher amounts of aluminum and lower phosphorus than natural, which can be seen from the above analysis.  My colleague was also kind enough to submit specimen #4 for analysis, it should be obvious from the data table that this specimen is not turquoise at, but rather dyed magnesite.  The specimen contains only trace amounts of copper and phosphorus with greater than 20% magnesium as MgO.  One should note the high amounts of barium and vanadium, which is also very interesting.  Barium is typically found naturally as the mineral barite, which is barium sulfate, however this specimen has very low concentrations of sulfur and contains no barite.  It is likely that both barium and vanadium are products of dye used as a colorant for this piece.  If this bead was soaked in solvent or broke in half, it would certainly reveal a white interior.  The final specimen, #5, is the most obvious out of the five samples analyzed.  Typically, one would not submit a sample such as this one for analysis, it lacks polish, has a very low density, and has an unnatural look and feel to it.  This cab was removed from a vintage costume jewelry piece, and you might have guessed it right that it is composed entirely of plastic.  Nevertheless, I wanted to share a chemical analysis of it, as it is beneficial to our discussion on treatments and faux turquoise.  The analysis shows less than 3% total, which is mainly titanium and some trace metals.  Like epoxy plastic is a polymer composed of carbon, which is not detected by XRF analysis.  The presence of titanium is expected, as titanium oxide is a common pigment used in paints and dyes as a colorant. 

With the onset of many types of turquoise available on the market and the wide array of treatments I hope you find this information useful and informative.  As a consumer looking to purchase gem grade turquoise one can never be too careful and should always buy from a reputable dealer.  Unfortunately, even rare high end vintage jewelry pieces are being counterfeited and sold as the real thing.  They can be crudely cast and affixed with treated turquoise at low costs, before they are resold to the unwary buyer for hundreds to thousands of dollars.  Most of these stones can be detected without chemical analyses, but some methods to do so are destructive.  Certainly, stabilized material can be detected by the chemical odor given off when the sample is cut or ground on a lap wheel, even igniting the sample with a flame will yield similar results.  Zachery treated stones can be much more difficult to detect, and unfortunately can only be confirmed with a chemical analysis.  Sometimes the Zachery treatment is obvious with visually apparent extreme coloration, but most times it is not.  Porcelain treatment can sometimes be observed with the waxy look and feel, and confirmed by the lower weight of the material, but again sometimes the differences can be subtle.  Fortunately, the identification of imitation turquoise is fairly easy as dyes can be removed by soaking the sample in acetone or a similar solvent or cutting the material to reveal the interior, but both are destructive means.

 I have decided to provide two options available for the XRF testing method, which includes the origin XRF analysis that provides a comprehensive report of mineralogy, properties, origin, and grade, as well as a simplified one that provides only the chemical analysis that is in the data table above.  The comprehensive origin XRF analysis contains a considerable amount of information for $65, it should be used to identify and grade seemingly valuable material.  The simpler XRF analysis is $30 and it should be used to identify if the stone is natural with no enhancements.  Sometimes, after acquiring the simpler analysis, one may wish to later acquire the comprehensive report with origin and grade.  As such, since the XRF analysis has already been ran, for another $35 to make up the balance to $65 the report can be upgraded to the comprehensive one at anytime.  If you wish to submit samples, please contact West Texas Analytical Laboratory at 580-977-6951 or feel free to submit requests at wtalab.com.

As Mike Ryan would say be well, be happy and always wear your turquoise.   

Neil Ray