New Mexico Mineral Symposium — Abstracts

What turquoise is seems to be a matter of perception rather than fact. Turquoise to a geologist is a hydrous cupric aluminum phosphate, CuAl₁₆(PO₄₎₄(OH)₈.5H₂0, with additional trace constituents of iron and silica. Turquoise to a consumer is a traditional semiprecious stone, strong in color, that is hard enough to polish and not crumble, dense enough to not absorb skin oils when worn, and plentiful enough to be available at affordable prices.

Unfortunately turquoise will never meet all of these criteria. Many forms of imitation and adulteration have been used to capitalize on an insatiable demand for this misunderstood gem. So, to an opportunist, turquoise is an abundant commercial material of acceptable blue color and natural-appearing matrix, capable of retaining these qualities for the few months needed for marketing. Price should be negligible and would increase only if an observable but unverifiable quantity of genuine turquoise is incorporated in the material. More valuable, in increasing order, are materials that will pass routine quantitative analysis, genuine stone temporarily doctored with oils and sealer, permanently doctored polymerizations and silica-dons, and finally most expensive and least available, the unadulterated natural stone.

In an attempt to protect the public from opportunists, while still allowing producers of altered or imitation stone to enter their legitimate place in the market, the New Mexico legislature passed the Indian Arts and Crafts Act of 1973, revised 1978 (sections 30-33-3 through 30-33-8). Though the lawmakers' understanding of the technical aspects of the problems might have been limited or even erroneous, their intent I believe was apparent. Subclass K reads:

"unnatural turquoise" means any substance which is not natural turquoise, including:
(1) "stabilized turquoise" which means turquoise which has been chemically hardened but not adulterated so as to change the color of the natural mineral;
(2) "treated turquoise" which means turquoise which has been altered to produce a change in the coloration of the natural mineral;
(3) "reconstituted turquoise" which means dust and turquoise particles which are mixed with plastic resins and are compressed into a solid form so as to resemble natural turquoise; and
(4) "imitation turquoise" which means any compound or mineral which is manufactured or treated so as to closely approximate turquoise in appearance.
All terms in this discussion that appear in quotes refer to this legal description.

In 1985, Bob North, Mineralogist for the New Mexico Bureau of Mines and Mineral Resources, and I collaborated to establish laboratory procedures for fractionating turquoise specimens in accordance with the Arts and Crafts Act. The logic we used in testing a specimen hinged on my theory that color and hardness depended on complete hydration, turquoise being a hydrated phosphate. The insufficiently hydrated forms of turquoise known as chalk are more abundant and less expensive than high-grade natural. Processes to regain hardness and color were what we needed to target when testing genuine turquoise for alteration.

 In practice, a piece of chalk will return to its fully hydrated color when saturated not only with water but also with oils, waxes, and most other polymers. Polymerization or color stabilizing (trade jargon coined, I believe, by the L. W. Hardy Company of Kingman, AZ) is a laboratory procedure incorporating clear catalyzed resins that harden and permanently seal turquoise in the rough. I believe this process is recognized as "(1) stabilized." A specimen containing organic compounds would definitely be unnatural but may lack the permanence and hardness of stabilization. For more insight, a carbon-hydrogen analysis was run on our fifteen samples and the results are presented at the end of this article. Several processes are identifiable by their carbon content.

Fracture-sealed turquoise is unfortunately not addressed specifically by the Arts and Crafts Act. It is sold more often than not as natural but in reality is a poor man's stabiliza¬tion of cut or finished stones. Unlike fracture-sealed vitreous gems, porous turquoise assumes the treatment generally and will gain rehydrated color levels. These in-shop formulations are usually adhesives or lacquers that harden once the carrier solvents evapo¬rate. The compounds tend to be ultraviolet sensitive and degrade with time. Our carbon analysis of fracture-sealed stones indicates carbon content much less than stabilized samples and about twice that of oiled samples.

In (2) "treated turquoise," fully hydrated color levels can be exceeded and slightly off-blue tints can be corrected with the use of dyes. These processes are usually used in conjunction with polymerization and are quite difficult to detect because of the fixing qualities of the polymer. Visual examination for indications of dissimilar concentrations, such as dye line and spots, and a good eye for unnatural shades have proven the most effective tests. Microscopic inspection of an oil infusion of a pulverized sample has netted conclusive results in some instances. In the case of a fairly new secret process, known as natural enhanced, dyes were not analyzed because the normally corresponding organic additives were absent. The sample was erroneously classed as natural. In discussions with the owner of the formula it was divulged that an electrical process on hard chalks was used. I then tried oxalic acid to bleach dye stuffs on the assumption that copper phosphate or a similar mineral tint was either being a) electrowinned into the crystalline lattice of the stone, or b) valence bonded to sodium silicate (silica occurs naturally in turquoise), which was being hardened by an electrical field rather than with electrolytes. The results were positive, a bleached spot at the acid contact, and conclusive, similar tests on natural and stabilized specimens were negative. Carbon content places the natural-enhanced process lower than the fracture-sealed process but not notably higher than unaltered stones polished with wax- or oil-base polishes. These polishes are used extensively and impart a slight color gain on only the most absorbent pieces of natural turquoise. In contrast, saturating the stone with oils and waxes yields strong color gain but imparts neither hardness nor color permanence.

"(3) Reconstituted turquoise" has evolved into two major types. The first is naturally occurring nuggets of chalk compressed and then bound by stabilizing polymers. The value of this material is based on availability of a genuine-turquoise base component. The second type is reconstructed powders. Even with the use of dyes, stabilized turquoise powder becomes an unappealing gray. A pure, white mineral filler, aluminum hydroxide (Al(OH)₃), must be used in substantial proportions before a dyeable mix is attained. This compounded substance will be passed as turquoise by most assay labs. The content of filler can be as much as 100% as in the case of the German imports sold as synthetic or genuine reconstruct¬ed. In our procedure x-ray diffraction was run first to quickly detect most imitations. If a sample contained aluminum hydroxide, it was classed as imitation and no more tests were run.

"(4) Imitation" products frequently can be detected by visual
examination, circumventing the need for further analysis. In the absence of readily available detection procedures, imitation has been lucrative and still spawns new formulations, as confirmed by Dr. Klein of the University of New Mexico. He had been sent a specimen of turquoise from Germany that had a beautiful lustrous deep Persian color and gemmy homogenous grain. On analysis, it was a carbon-calcium-nitrogen compound with minor amounts of phosphoric, titanium, and aluminum oxides.

Back in 1985 our intent was to implement the enforcement of a much-needed law. Today my hopes are that the State will fund continuing research in an effort to discourage fraudulent marketing. Informed, educated consumers help this effort by becoming more aware of what is being sold. 

treated: A catch-all phrase that no longer indicates any particular process, an irrelevant term except for legal description