Lithify is a verb that essentially means to turn into stone or harden. While it's a commonly used term in geology, there are several different words you can use to describe this process. Some of these synonyms include fossilize, petrify, solidify, mineralize, harden, and calcify. Each of these words describes the process of turning something into a rock-like substance, whether it be a bone, wood, or sediment. Choosing the right word will depend on the context and what you're specifically referring to, but it's good to have a few different options to choose from for the sake of variety and clarity.