What is "Fool's Gold"?

By GovMint :

"Fool's Gold" is the common nickname for the mineral pyrite or iron sulfide (FeS₂). Fool's gold's metallic luster and pale brass-yellow coloring give it a superficial resemblance to gold, historically leading to confusion among prospectors and miners. Unlike real gold, pyrite is brittle and will shatter when struck, whereas gold is malleable and can be shaped without breaking.

Sources Used:

https://www.thermofisher.com/blog/mining/pyrite-the-real-story-behind-fools-gold/