Lab Notes Gems & Gemology, Fall 2013, Vol. 49, No. 3

Turquoise with Fingerprint Pattern

IMG-FA13-LN 154338 636x358
Figure 1. A few of the natural, untreated turquoise beads in this necklace display a fingerprint pattern. Photo by Don Mengason.
The Carlsbad laboratory recently identified a natural, untreated turquoise bead necklace with an interesting feature. The bluish green beads displayed a natural turquoise structure with thin black matrix veining (figure 1). During microscopic examination, a few of the beads showed a less-saturated greenish discoloration pattern resembling fingerprints. The bead selected for testing had a spot RI of about 1.60 and fluoresced weak bluish green under long-wave UV, with no short-wave fluorescence. To rule out polymer impreg­­nation treatment, we collected an infrared (FTIR) spectrum. It showed no peaks in the polymer regions, ruling out polymer-impregnated turquoise (K.S. Moe et al., “Polymer-impregnated tur­quoise,” Summer 2007 G&G, pp. 149–151). The unusual pattern may have been the result of glue or another contaminant present during the initial handling or drilling of the beads, preserving actual human fingerprints. With time and wear, the discoloration of the beads made the fingerprints more distinct (figure 2).

IMG-FA13-LN 154336 636x358
Figure 2. The light area shows a discoloration in the form of a fingerprint pattern. Photo by Nathan Renfro.
Untreated turquoise is porous and often absorbs skin oils and other contaminants, making the color more greenish. As a result, turquoise is commonly treated to enhance its appearance and protect it from discoloration. Most of the product on the market, especially lower-quality material, has been wax- or polymer-impregnated to improve its durability (S.F. McClure et al., “Gemstone enhancement and its detection in the 2000s,” Fall 2010 G&G, pp. 218–240).

This interesting piece serves as a useful example of why turquoise is routinely treated. Simply handling it can affect the color and appearance over time, as illustrated by the fingerprint pattern preserved in this piece.

Tara Allen and Amy Cooper are staff gemologists at GIA’s laboratory in Carlsbad, California.