perspiration

The Incas believed that gold was the sweat of the sun.

potato

"The Incas developed the potato, which is a tasty hybrid of poisonous plants."
- Dan Cruikshank

Image source here.

only worth what others will exchange for it

To ancient Incas gold was the sweat of the sun and silver the tears of the moon whose value was aesthetic. Labour was the Inca unit of value.

An earlier reference here.