Have you ever seen a fossil that looked as though it were carved from pure gold? Chances are you were looking at an example of pyritization [pyritisation if you're a fan of British spelling]. It may not be made of precious gold, but it's an entirely natural fossil formation.
Pyrite, often called Fool's Gold, is an iron sulfide, and a very common mineral. Because of its shiny, metallic golden color, it has often been mistaken for true gold. It is brittle, and sadly subject to decay in moist environments, making it useless for most practical purposes.
Nonetheless, it makes beautiful decorative mineral specimens!
Pyrite replacement of fossils is often caused by bacteria, in a process called permineralization. Since it requires both aerobic bacteria (which need oxygen to survive) and anaerobic bacteria (which live in places with little to no oxygen) to complete the process, pyritization can only occur in the levels of sediment near the interface between the two zones - and the sediment must have just the right amount of iron, not too much and not too little.
When the body of the unfortunate soon-to-be fossil is first buried, sulfate-reducing anaerobic bacteria begin to consume its organic material, producing sulfide.
The high concentration of iron in the sediment converts this to iron monosulfide, which is then oxidized into pyrite by aerobic bacteria. This pyrite is deposited onto the surfaces of the decaying organism, forming sparkly pyrite.
Fossils formed in this way can preserve the forms of the soft tissue, a rare event in the fossil world, making them particularly useful in scientific research.
Some famous examples of pyrite-replaced fossils occur in the Beecher's Trilobite Bed, in New York State. Trilobites from this location have been replaced by pyrite, producing a three-dimensional fossil that replicates the internal anatomical structure of the trilobite's organs.
Unfortunately, pyrite can be subject to "pyrite disease", a condition in which your beautiful piece of pyrite oxidizes and crumbles to ugly powder. Some pyrites are more stable than others. Humidity can cause decay, so keep your specimens clean and dry. If you found them near the seashore, wash them thoroughly to remove any salt, and dry carefully before storing.
Some people recommend giving them regular baths in mineral oil, but I cannot vouch for that one way or another. If you don't mind oily rocks, perhaps you can give it a try and inform me of the results.
If you'd like further reading on the subject, I recommend this book: Exceptional Fossil Preservations