Reversible Twinning in Neodymium Pentaphosphate
Unusual synthetic gem materials are often the result of experimental crystal growth for industrial applications, including use in the laser industry. One such experimental synthetic that was produced in a very limited quantity is neodymium pentaphosphate, with the chemical formula of NdP5O14 (Winter 1997 Gem News, pp. 307–308). Examined using polarized light, it displays very prominent lamellar twinning. Lamellar twinning is a regular parallel repeated reversal in growth that causes the twinning planes to appear as alternating dark and light bands when observed in polarized light (see above). The material’s most unusual property is that when slight pressure is applied perpendicular to the twinning planes, the twinning reverses so that the dark areas can become light, and the light areas become dark. Interestingly, the reversal is also accompanied by a weak clicking sound. The unusual elastic twinning of this material makes it exceedingly difficult to cut into a gem, though a 5.33 ct stone was faceted by Art Grant of Coast-to-Coast Rare Gems in the 1990s. To the authors’ knowledge, no other material displays this interesting property.
