Turquoise: Blue of the sky & green of the sea
Gagan Choudhary, FGA
Part 2: Treated Turquoise
This is second part of the 3-part series on turquoise, covering its treatments, their detection and disclosure.
Recap from part 1: Turquoise contains microscopic crystals forming a solid mass. If the crystals are packed closely together, the turquoise is less porous, and hence a finer and smoother texture. Turquoise with loosely packed crystals has higher porousity with coarser texture. Both porousity and texture affect the appearance and durability of turquoise. Lesser the porousity, smoother the texture, higher the durability and better toughness, and vice versa. Fine textured turquoise has an attractive waxy to sub-vitreous lustre when polished, while a coarse textured turquoise appears dull after polish. Therefore, turquoise with low porousity and finer texture commands higher price compared to that with high porousity.
Most of the turquoise when mined appears chalky and whitish when mined, due to its porous structure. This is mainly due to scattering of light from microscopic crystals forming the turquoise nodule. Such chalky turquoise is unsuitable for cutting and polishing as it simply crumbles during the process and is also highly susceptible to body oils and other environmental impacts, which often converts blue turquoise into green. Therefore, due to turquoise’s inherent structure, most of the turquoise present in the market require some or the other form of treatment. It is estimated that less than 10% of mined turquoise can be used in jewellery without any treatment. However, in experience of the author during past 20 years at the GJEPC-Gem Testing Laboratory, over 99% of turquoise tested were found to be treated by some or the other process.
Figure 1: Polymer treated 'stabilised' turquoise (top, left), proprietary 'zachery-treated' turquoise (top, right) and dyed turquoise (bottom, right).
THE KEY: FILL IN THE PORES
Being porous in nature, the most common and effective treatment on turquoise is filling up of pores with organic substances such as colourless to near-colourless oils, wax, polymers, commonly referred to as “stabilisation process” or filling with inorganic silica or potassium-based compound, although, use of inorganic compounds is not commonly encountered.
A common term being used in the trade is “stabilised turquoise”, which basically refers to a turquoise filled with colourless polymer or wax. As a result of filling up of pores, porous and fragile turquoise gets much more stable, allowing its cutting and polishing; such filling process also allows the light to reflect uniformly, thereby enhancing turquoise’s colour and lustre – giving turquoise a glossy look. Filling of pores also prevents loss of moisture and oxidation of iron from Fe2+ to Fe3+, which may cause the colour to change from blue to green. Stabilisation using oil and wax is much less durable as compared to polymers / resins. Use of these two substances will degrade the turquoise turning them yellowish, even if they are not used and just kept in storage. However, when freshly treated, by whichever material, all turquoise appears similar and it becomes difficult to correctly describe the type of treatment performed, without the use of advanced techniques like Raman or infra-red spectrometry.
The treatment process typically involves the use of high-pressure vacuum chamber where heated resin is introduced into the pores of turquoise at pressures of up to 100 bars. Use of high-pressure ensures better and thorough penetration of resin into the pores. Depending on the type of resin used, filled turquoise are placed in ultra-violet light or electrons to cure the resin and its stabilisation.
The Zachery Treatment
A proprietary treatment developed in late 1980s involve the use of inorganic substances to fill the pores of medium to high quality turquoise – the purpose and result remains improvement of colour, lustre, and durability. The treatment involves more advanced process of filling and because of the use of inorganic substance, treated turquoise with this process are more durable compared to those treated with polymers / resins. Due to its proprietary nature, exact details of the treatment and its process are not available, however, advanced spectroscopic analyses at molecular levels reveal the use of potassium-based compound, which also forms the basis of its detection.
COLOUR TREATED OR DYED TURQUOISE
Chalky and pale blue turquoise which do not result in desired blue colour after ‘stabilisation’ process mention above, are treated with coloured wax or resins. The method of treatment remains the same, but a colouring agent or dye is added to the base compound. The treatment adds blue colour to the otherwise ‘pale’ turquoise, providing uniform colouration.
Detection of treated turquoise involves a multi-step criterion, starting with microscopic study of texture, veins, cavities, crevices, etc. to establish presence or absence of fillers, followed by examination under UV lamp. Detailed analyses are performed by Raman and infra-red spectrometers to determine the presence of wax or polymer, while EDXRF is used for chemical analysis.
Turquoise with even blue colouration and glossy look is the first indication of presence of treatment. However, a treated and untreated counterpart should be compared to observe the difference and be followed by further analyses.
In some cases, deeper colour is observed along the rim or outline of the stone due to the limited penetration of the filler, however, one must be careful while drawing any conclusion on this basis. In experience of the author, such colouration may also be caused temporarily when turquoise rough is stored in water, or during the process of cutting and polishing which also uses water, or application of oil during the polishing process.
Figure 2: An illustration of
effect of filler on turquoise. The turquoise specimen displays deeper colour on a spot
applied with baby oil, while rest of the area appear much lighter.
In addition to the smooth surface, cavities, pits and fissures may display the presence of filler substance. This is generally visible as transparent areas against the opaque surrounding turquoise. Improper filling may also result in gas bubbles to trap in large cavities, pores, or fissures. In case of coloured substances, these cavities / pits display blue colour concentrations, suggesting presence of dye.
Figure 3: Filled cavity in a turquoise impregnated with colourless polymer. In some cases, hemi-spherical cavities may also be visible (right image).
Figure 4: Filled veins and cavities in a turquoise impregnated with coloured polymer. Also note deep blue colour concentrations.
Depending on the type of filler substance used for impregnation or stabilisation, treated turquoise usually display moderate to strong blue or greenish blue under UV light, as compared to the untreated counterparts. Reactions under longwave are stronger than shortwave.
Figure 5: Polymer-treated turquoise (right) display stronger blue fluorescence under long-wave UV light, compared to natural counterpart (left).
Micro Raman spectroscopy is one of the most powerful tools to conclusively detect the presence of organic substances such as polymer, resin or wax. It helps to perform analyses on a particular spot or vein and hence information can be collected from different areas of a sample. A collection of reference samples is however necessary to differentiate between treated and untreated counterparts.
Figure 6: Raman spectra of natural (black trace) and polymer-treated turquoise (blue trace).
Like Raman spectroscopy, infra-red spectroscopy is another powerful tool in detection of organic substances. Separation of wax and resins can conclusively be established by considering the position of features.
Figure 7: Infra-red spectra of
natural (black trace) and polymer-treated turquoise (blue trace).
Chemical analysis is a powerful tool in detection of the proprietary ‘Zachery’ treatment. Treated stones display high concentration of potassium, while it is either absent or present only in minor quantities in untreated counterparts.
Figure 8: ’Zachery-treated’ turquoise (top) displays high concentration of potassium, which is absent in natural turquoise.
All treated turquoise, whether stabilised, dyed or ‘Zachery-treated’ require disclosure at the time of sales. At the IIGJ-Research & Laboratories (formerly, GJEPC-Gem Testing Laboratory), all turquoise treatments are routinely disclosed on the identification reports. Following are the comments given on identification reports for treated turquoise:
Due to its inherent structure, most of the turquoises require treatment enabling cutting and polishing, enhance surface lustre, appearance and durability. However, each treatment type requires clear disclosure at time sale.
Date of Publishing: 29.08.2022