… is not gold
– a well known Shakespearean quote from The Merchant of Venice. It’s certainly true in geology, where there are any number of gold coloured minerals.
Where I live, there are large quantities of “fool’s gold” or iron pyrites – actually a rather brassy looking mineral, which is iron sulphide FeS2. It’s quite heavy and collects in drifts on the beach between Charmouth and Lyme Regis. If you are patient, you may find a perfectly pyritised ammonite (usually tiny), but it occurs in all sorts of forms: microcrystalline – sometimes replacing fossils as you can see here, as clusters of larger cubic crystals, or smooth rusty-looking nodules with a fibrous internal structure. It was formed by anaerobic decomposition of organic matter in iron-rich sediments on the sea floor, and is common throughout the lower Jurassic sequence near Lyme and Charmouth.
If you do find good specimens, enjoy them while you can, as, taken away from their natural habitat, they tend to oxidise and crumble away into a sulphurous powder.