Editor’s Note
This article explores the enduring appeal of turquoise, one of history’s most cherished gemstones, and examines the long-standing practice of creating and using its imitations.
Turquoise is one of the oldest known and widely used gemstones throughout history and across various cultures and civilizations. This gemstone is an opaque cryptocrystalline mineral that occurs in blue, blue-green, and green colors, with the chemical formula CuAl₆(PO₄)₄(OH)₈·4H₂O. Due to its beautiful color, high popularity, and unique significance to people, there are numerous simulants of turquoise available in the market today. The use of turquoise simulants dates back at least 6000 years in Egypt and among the Roman civilization (1). Some minerals, such as chrysocolla, microcline, lazulite, serpentine, odontolite, variscite (2), and prosopite (3) can be mistaken for turquoise due to their color and appearance.
In addition to natural and mineral imitations, other turquoise imitations, such as dyed calcite, halite, turquoise powder mixed with glue, glass, porcelain, plastics (2), and colored agate (often referred to as “turquoise agate”), which has recently been introduced to the market as a turquoise substitute (4), can also be mentioned. Natural turquoise is usually highly porous. Various treatments, such as saturation, dyeing, or the Zachery process (5), can increase its strength and reduce its porosity. However, it is not difficult to distinguish between filled turquoise with resin and untreated turquoise (6). Since the early 1970s, artificial turquoise has also entered the market extensively (1).
The variety of turquoise gemstones available and the potential for misuse by forgers have increased the importance of identifying natural types of this gemstone from others. Today, the use of techniques that cause minimal contact and damage to the gemstone is a priority for gemologists.
Bernardino and associates (7) utilized Raman spectroscopy as a non-destructive method to identify and distinguish various turquoise samples with different appearances. This type of spectroscopy has been recognized as an effective method for identifying provenance and determining gem treatments in gemological laboratories since the late 1970s (5). Distinguishing between natural and treated turquoises (8), as well as tracing the provenance of archaeological turquoise (9–11), are other beneficial outcomes of Raman spectroscopy in turquoise studies.
On the other hand, ultraviolet-visible-near infrared (UV-vis-NIR) spectroscopy, in both reflection and absorption modes, is another non-destructive and non-invasive method for identifying genuine turquoise from fake, dyed, and resin-filled specimens (12,13). Fiber optic reflectance spectroscopy (FORS) is a useful alternative to other classic gemological techniques and provides beneficial results for distinguishing and identifying natural gemstones from their simulants (14–16).
Spectral imaging, another non-destructive technique for studying gemstones, has been developed over the past 60 years and has yielded effective results in the analysis of paintings, textiles, written documents, and the conservation of artworks (17–24). Among these techniques, precious and semi-precious stones are frequently analyzed using the ultraviolet-induced visible luminescence (UVL) imaging method, which relies on the observation of visible luminescence produced under UV-A and UV-C ultraviolet light (25,26).
An examination of natural turquoise using ultraviolet light reveals yellow-green to light blue luminescence under long wavelengths, while short wavelengths exhibit neutral luminescence (27). This visible luminescence is not only beneficial for the initial assessment of the gem but also offers valuable insights regarding its treatment (28).