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This is third and final part of the 3-part series on turquoise, covering its imitations and their detection.

Recap from part 1 and 2

Turquoise contains microscopic crystals forming a solid mass. If the crystals are packed closely together, the turquoise is less porous, resulting in 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.

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.

Due to increasing popularity of this gem material there is a significant rise in number and types of imitations being presented as turquoise in recent years, both rough and fashioned. For ease of understanding, these imitations are classified into two groups as follows:

Group 1: Mainly consists of colour-treated (dyed) natural materials, such as magnesite, howlite, alunite, chalcedony and dolomite; dyed magnesite being the most commonly encountered material.

Group 2: Comprises man-made materials, including composites made from turquoise and/or magnesite grains and chips, in various patterns, such as, with or without black/brown veins, with or without pyrite/marcasite, and with or without metallic (Zn/Cu) flakes/veins. Another common imitation-type in this group includes reconstructed or ceramic materials composed of mineral powder, especially of turquoise (often called ‘synthetic’ in trade), gibbsite or barite, pressed and bonded in a polymer, with a colouring agent; these ceramic-imitations are also available in various patterns, as described above for composite-imitations. Further to composite- and ceramic-imitations, translucent to opaque glass is also occasionally encountered.

GROUP 1: COLOUR-TREATED (DYED) NATURAL MATERIALS

GROUP 2: MAN-MADE / ARTIFICIAL MATERIALS

8. Ceramic-imitation turquoise with and without veins (matrix). Such materials are created using grains or powder of turquoise, gibbsite or barite.

IDENTIFICATION AND SEPARATION

Observational Features

Textural observations under a microscope remain the most important tool, especially while determining presence of colour concentrations in natural dyed materials, or deep crevices with polymer veins or randomly oriented mineral grains in composites, or fine granular texture with ‘blue-dots’, commonly referred to as ‘cream-of-wheat’ effect in synthetic (ceramic-imitation) turquoise.

Careful observations of structural features under a microscope or loupe can spot the difference between natural and various artificial materials described above, however, natural dyed materials can pose challenges.

Spectroscopic Features

Non-destructive spectroscopic methods of analyses, such as infrared, absorption or Raman, along with EDXRF analyses are the most useful tools to characterize and separate natural turquoise from treated counterparts or its imitations. In routine, the IIGJ-RLC (formerly, GJEPC-Gem Testing Laboratory) uses a combination of qualitative EDXRF analysis, Raman spectroscopy and microscopic observations to identify and separate various types of turquoise imitations.

CONCLUSIONS

Availability of wide range of imitations and simulants has created a lot of confusion within the turquoise trade, as most of the discussed materials cannot be identified and classified with classical gemmological testing. Since most of the natural turquoise available in the marketplace is ‘stabilized’ using colourless or coloured polymers, gemmological characteristics such as RI and SG are not useful for their identification. However, a combination of observational features under a microscope, qualitative EDXRF analyses and Raman spectroscopy helps to identify and separate turquoise from its simulants and imitations.