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.
I've read some accounts from conquistadors who described some Incan objects that were apparently never captured by the Spanish. One was the "Golden Chain of Huascar" which was supposed to be a couple of hundred feet long and made from gold (otherwise I suppose it would have been called the "silver chain of Huascar"). This chain was said to have been hidden in the bottom of a lake to keep it from Huascar's brother, Atahualpa, during their power struggle.
Some of the buildings in Cuzco were said to be clad in silver and gold foil, before the Spaniards stripped it all off. Must have been quite a sight.