Permineralization is one of the ways in which a plant or animal can be turned into a fossil. During this process, mineral rich water seeps into the void spaces of an organism, leaving deposits of minerals behind that slowly build up, creating a cast of the organism. In some cases, the entire plant or animal may be turned to stone this way, in which case the resulting fossil is said to be petrified. Many museums have examples of fossils that have been preserved through this method, and people can sometimes find such fossils for sale at rock shops and hobbyist stores as well.
This process is very uncommon. Billions of organisms have died on Earth over the course of geologic history, and only a fraction of these organisms have been fossilized because nature has evolved a very elegant and efficient system for recycling dead plants and animals. For permineralization to occur, an organism must be rapidly covered in sediment shortly after death. Many petrified fossils are marine organisms that fell to the bottom of the ocean and were covered in sediment.
The permineralization process is very slow. It happens as water seeps through the sediment that covers an organism; the sediment helps keep the organism intact, and the mineralized water slowly works its way into the remains. Depending on the type of minerals involved and the conditions, petrification may result in incredibly detailed preservation, allowing people to differentiate individual cells in the organism, or it may create a more rough cast.
When petrified fossils contain primarily silica, archaeologists say that they have undergone silicification. Silicification is an excellent preservation technique that sometimes retains immense detail. When sulfur is involved, the process is called pyritization. Pyritization happens most commonly to marine organisms. It is also possible to find some excellent examples of carbonate mineralization in places like coal mines.
In many cases, permineralization only preserves the mineral tissue in the organism, like bones, teeth, and shells. In other instances, it fills a void left behind in sediment by an organism which has since rotted; this commonly happens with fossilized leaves. In very rare cases, part or all of the soft tissue is preserved, allowing people to get a glimpse into a body that may be millions of years old to learn more about how it worked.