'Fool's gold' is the common name of:

Iron pyrite, or pyrite, is a mineral made of iron disulphide, with the formula FeS2. It has a shiny, metallic, brass-yellow colour that looks very much like gold. Many inexperienced miners and prospectors, especially during gold rushes, mistook it for real gold. This is why it is called "fool's gold".

Pyrite can be told apart from real gold in several ways:

• Gold is soft and can be bent or scratched easily with a knife; pyrite is hard (about 6-6.5 on the Mohs scale) and brittle.
• When rubbed on unglazed porcelain (a streak test), gold leaves a yellow streak, while pyrite leaves a greenish-black or brownish-black streak.
• Gold is very heavy (density about 19.3 grams per cubic centimetre); pyrite is much lighter (about 5 grams per cubic centimetre).
• Pyrite often forms cube-shaped crystals with straight edges, while gold usually occurs as nuggets, flakes or grains.
• Pyrite gives off sparks when struck against steel; the name "pyrite" comes from the Greek word pyr, meaning "fire".

Pyrite is not a major source of iron, because extracting iron from it is difficult and produces sulphur dioxide. It is mainly used to produce sulphur dioxide for making sulphuric acid. Interestingly, small amounts of real gold are sometimes found inside pyrite deposits.

Iron ores used to make iron and steel are mainly haematite (Fe2O3), magnetite (Fe3O4), limonite and siderite.

Why not the other options?

• ✗Copper pyrite (chalcopyrite, CuFeS2) is the main ore of copper; it also looks yellow but is not usually called fool's gold.
• ✗Galena (PbS) is the main ore of lead.
• ✗Cinnabar (HgS) is the main ore of mercury.