Species / Variety: Orthoclase/Rainbow Moonstone
Origin: Madagascar
Color: silvery-white body color with strong Peach ghost sheen and multi rainbow colors
Carat Weight: 3.31 cts.
Shape/Cut: Custom Rectangle - Square Cabochon
Size: 10.7 X 9.2 MM  Depth: 4.2 MM
Clarity: Translucent
Treatment: No treatments, 100% Natural!

Zambia and Madagascar (figure21).

The Madagascar stones were offered by Paul Dragone (Boston Gems, Boston, Massachusetts), who loaned a 4.41 ct sample to GIA for examination. Mr. Dragone first encountered the rough material at the 2011 Tucson gem shows. Initially, from 12 rough pieces weighing about 3 g each, he cut more than a dozen cabochons weighing up to 7.11 ct (18% yield). Later he obtained additional rough material, from which he cut 35 cabochons that averaged approximately
 3 ct each.
 
The gemological properties of the 4.41 ct cushion-shaped double cabochon were: color—near colorless, displaying orange, yellow, green, blue, and violet adularescence; diaphaneity—transparent; spot RI—1.56; hydrostatic SG—2.69; andfluorescence—weak white to long-wave UV radiation, and weak red to short-wave UV. No absorption lines were visible with the desk-model spectroscope. These properties are consistent with those reported for plagioclase (M. O’Donoghue Gems, 6th ed., Butterworth-Heinemann, Oxford, UK, 2006, pp. 259–269), and are also similar to the rainbow moonstone (andesine–labradorite) recently reported from Zambia. Microscopic examination revealed polysynthetic twinning, a typical feature of plagioclase, as well as small “fingerprints” and numerous scattered reflective films. Many of these films were oriented along twinning planes.
 
EDXRF spectroscopy showed the presence of Al, Si, Ca, and Na (consistent with plagioclase), as well as traces of Fe, Sr, and Ba. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) data further placed it approximately halfway between the plagioclase end members albite and anorthite, on the andesine/labradorite border with a compositional range of Ab48.6–51.7An49.8–46.8Or1.6–1.5.
 
The rainbow colors were best viewed against the dark background afforded by darkfield illumination (figure 22), but they were also easily seen in transmitted light. In addition, a billowy adularescent effect was produced by a fiber-optic light source positioned over the stone, with orange predominating over other colors, and was best seen when the light was aligned perpendicular to the twin planes. The twin planes, on the other hand, changed from dark to light lines depending on the light orientation.

Figure 22. This 4.41 ct rainbow moonstone displays a billowy adularescent effect when the light source is moved over the stone (left, magnified 8×). On the right (11×), in darkfield illumination, various spectral colors are visible as well as polysynthetic twinning. Photomicrographs by Claire Ito.

When the stone was viewed down its length, particularly under magnification with the fiber-optic light held close to the surface, it appeared hazy, consistent with the light scattering that accompanies adularescence.
 
Traditional moonstone (orthoclase) displays a white to blue adularescence thought to be caused by Rayleigh scattering 
of light from the exsolution of albite within the Kfeldspar structure (E. Fritsch and G. R. Rossman, “An update on color in gems, part 3: Colors caused by band gaps and physical phenomena,” Summer 1988 G&G, pp. 81–102).

Labradorescence is an interference phenomenon that may be exhibited by labradorite due to the diffraction of light from exsolution lamellae of varying Ca content, potentially producing many spectral colors; the relative  thicknesses and differing refractive indices of the lamellae determine which colors are visible (Fritsch and Rossman,1988). The present moonstone (andesine-labradorite) exhibits a rainbow-colored effect that results from a combination of labradorescence and adularescence.