Feature Gems & Gemology, Winter 2006, Volume 42, No. 4

Leopard Opal: Play-Of-Color Opal in Vesicular Basalt from Zimapán, Hidalgo State, Mexico

 “Leopard opal” consists of vesicular basalt impregnated with play-of-color opal, and is known only from Zimapán, Hidalgo State, Mexico. The formation of this ornamental stone was made possible by an abundance of silica derived from the chemical breakdown of overlying volcanic ash layers, the permeability of the underlying basalt, and the presence of pores in the basalt of an aesthetically pleasing size. The even distribution and small size of the opal-filled vesicles makes the rock attractive when cut or carved and polished. Veinlets and irregular masses of play-of-color opal showing various bodycolors (red, white, and colorless to pale blue) have also been deposited along joints and fractures within the basalt flow. This opal deposit, which may have been worked in pre-Columbian times, has been explored only by a number of small test pits in recent years, and significant potential remains for its future development.