Sometimes folk add a coating of brass to increase the aesthetic appeal. Though this practice is frowned upon in paleontological communities.
Pyrite is a brass-yellow mineral with a bright metallic lustre. It has a chemical composition of iron sulfide (FeS2) and is the most common sulfide mineral. It forms at high and low temperatures usually in small quantities, in igneous, metamorphic, and sedimentary rocks.
When we find a fossil preserved with pyrite, it tells us a lot about the conditions on the seabed where the organism died. Pyrite forms when there is a lot of organic carbon and not much oxygen in the vicinity.
The reason for this is that bacteria in sediment usually respire aerobically (using oxygen), however, when there is no oxygen, they respire without oxygen (anaerobic) typically using sulphate.
Sulphate is a polyatomic anion with the empirical formula SO2−4. It is generally highly soluble in water. Sulfate-reducing bacteria, some anaerobic microorganisms, such as those living in sediment or near deep-sea thermal vents, use the reduction of sulfates coupled with the oxidation of organic compounds or hydrogen as an energy source for chemosynthesis.
High quantities of organic carbon in the sediment form a barrier to oxygen in the water. This also works to encourage anaerobic respiration. Anaerobic respiration using sulphate releases hydrogen sulphide, which is one of the major components in pyrite.
So, when we find a fossil preserved in pyrite, we know that it died and was buried in sediment with low quantities of oxygen and high quantities of organic carbon. If you have pyrite specimens and want to stop them from decaying, you can give them a 'quick' soak in water (hour max) then wash them off, dry thoroughly in a warm oven. Cool, then soak in pure acetone for a couple of days. Then soak in paroloid/acetone for a couple of days. Keep in a sealed container with a desiccant pack afterwards to keep them dry.