Microworld Gems & Gemology, Summer 2015, Vol. 51, No. 2

Bicolor Double-Eye Tourmaline


Bicolor tourmaline double cabochon displaying pink and white chatoyant bands on opposite sides.
Figure 1. Fashioned as a double cabochon and weighing 6.39 ct, this bicolor tourmaline from Newry, Maine, displays pink and white chatoyant bands on opposite sides. One side of this tourmaline is pink (left), while the reverse side is colorless (right). Even though the chatoyancy-causing growth tubes are only in the colorless portion, both sides exhibit a sharp cat’s eye. Photos by Kevin Schumacher.
The Summer 2009 G&G Lab Notes section (pp. 139–140) described a 5.44 ct tourmaline double cabochon that displayed a sharp, silvery white chatoyant band on one side and a deep golden brownish yellow eye on the reverse side. Microscopic examination revealed that this phenomenal optical effect was due to a combination of pronounced color zoning and growth tubes that were in precise crystallographic alignment.

This same phenomenon was observed in a double cabochon cat’s-eye tourmaline cut from rough discovered at the well-known pegmatite complex in Newry, Maine. This cushion-shaped cabochon, weighing 6.39 ct and measuring 11.31 × 11.12 × 6.39 mm, was pink on one side and essentially colorless to white on the opposite side. Consequently, its chatoyant bands had clearly different colors, one pink and the other a silvery white (figure 1).

As expected of a cat’s-eye gem, when two incident light sources (fiber-optic wands) were positioned side by side above this tourmaline, two separate eyes became visible. As the gem was rotated under and between the light beams, these eyes appeared to converge, closing to form a single chatoyant band and then opening, like a winking eye. This interactive phenomenon was visible on both sides of the double cabochon.

As with the previously reported example, microscopic examination revealed very strong color zoning, with a pink layer positioned parallel to a zone of near-colorless tourmaline, also known as achroite. The gem was fashioned so that the color layers were divided parallel to the girdle plane. The achroite layer appeared silvery white because of numerous parallel reflective growth tubes uniformly distributed throughout it (figure 2), while no growth tubes were present in the transparent pink-colored zone. Reflection of light from these parallel tubes caused the silvery white eye observed on the colorless side of the cabochon. The opposite side owes its pink chatoyancy to reflection from those same tubes inhabiting the near-colorless zone as the light is transmitted back through the pink transparent layer of tourmaline.

Growth tubes causing the chatoyancy in the bicolor tourmaline.
Figure 2. Growth tubes oriented along the c-axis and present only in the colorless portion are responsible for the chatoyancy in this bicolor tourmaline, observed using fiber-optic illumination. Field of view 3.59 mm. Photomicrograph by Nathan Renfro.
This is the second time we have encountered a tourmaline displaying different colors of chatoyancy on opposite sides of a double cabochon. In order to enjoy both eyes, such a stone should be set as a pendant in a simple bezel with a swivel, allowing exposure of either the pink or the silvery white cat’s eye as desired. 

John I. Koivula is the analytical microscopist at GIA in Carlsbad, California.

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