This is a classic 1960s-era silver specimen from the famous Batopilas Silver Mine in Mexico. Most of these were smelted for silver, it being a silver mine! Some relatively small portion was saved, and these sharp spinel-twinned crystals were seen when they were etched out of the calcite which had surrounded and frozen them in the deposit. This specimen is unusual because it was so carefully etched out, to leave some contrasting white calcite behind. It came to well known California collector and recluse, Hubert de Monmonier, who donated his vast collection to the University of Arizona long ago. It came out in legal deaccessions authorized by the university administration to raise money for a new museum building, around 2010. An old classic, in excellent form. The crystals reach to just about 3 cm.