From Lost Fables Wiki
Mercury is a shiny, highly reflective metal which is liquid at room temperature. In its raw form it is very bad to ingest, causing all sorts of neurological issues and taking a very long time to pass out of the body.
Mercury is mostly gathered as a byproduct of refining other ores, getting skimmed off the top of molten ore during the refining process. While this process is neither difficult nor expensive, the actual amount of mercury gathered as opposed to the level of demand for it makes it an uncommon good.
Mercury has relatively few uses compared to most alchemical reagents. Alchemically, mercury can be used with only a very few other reagents, and a base potion of mercury is merely extremely poisonous. As far as mundane uses goes, it is great for use in mirrors and thermometers.