Was the gold that the Conquistadores killed for a stone age artefact? Pre-Columbians in America also worked Copper, Platinum and Silver. Iron was also present along the North Pacific Coast.

https://en.wikipedia.org/wiki/Metallurgy_in_pre-Columbian_America#Northwest_Coast

http://www.pnas.org/content/105/13/5002.full

"The burial dates to 3776 to 3690 carbon-14 years before the present (2155 to 1936 calendar years B.C.), making this the earliest worked gold recovered to date not only from the Andes, but from the Americas as well. "

Have you seen Machu Picchu in picture or in person? Do you understand the engineering behind it? A stone fortress / temple on a mountain top, like a European Castle.