Didn’t the Spanish loot South America for like 300 years of gold and silver.
These cups do look like silver, but I can’t remember if the article sez what they are. Looking for gold and silver was the big attraction for the conquistadors, and the Bolivian silver mines were (maybe still are) the richest in the world. Colonial Spanish mining continued for 350 years, and only hit a bump, it sez here, during a worldwide silver glut in the 1890s. Tin mining also went on there, and extraction in other locales in the world probably caught on in large part because the entrance to the Potosi mines are at 13,000 feet elevation.
Pedantic sidebar — Butch Cassidy and Sundance Kid infamously fled to Bolivia because of the continuing mining activity. This resulted in their getting ambushed and killed by the Bolivian army. There has been romantic speculation that one or both of them escaped and returned under assumed names to the US. As it happens, the surviving letters from Butch all antedate the Bolivian ambush. I don’t think that’s a coincidence. Butch had at least one brother with whom he was often confused (and vice versa), and during his twilight years that brother apparently passed himself off as Butch from time to time.