Per the title Fool's Gold, the natural mineral is pyrite, an iron sulfide with the formula FeS2 that is often mistaken for gold.
Set in the fictitious town of Julian, New Mexico and filmed on location in Julian, CA, East of San Diego. According to Cannon's map, Julian is approximately 20 miles southeast of Corona on Highway 42. If one looks at a road map now there is no town on that highway between Corona and Ramon.
The robber's escape plane at the end is a 1964 Cessna 206 s/n 206-116. It is still airworthy and flying the skies of Eagle Creek, OR.