Diiodomethane is primarily used in organic synthesis as a reagent for various chemical reactions. It is often employed in the preparation of iodinated compounds, which are valuable intermediates in pharmaceuticals and agrochemicals. In analytical chemistry, it serves as a heavy liquid for density separations, particularly in mineralogy and geology, where it helps in the separation and identification of minerals based on their density. Additionally, diiodomethane is utilized in the formulation of high-refractive-index liquids, which are essential in optical applications such as microscopy and lens testing. Its unique properties also make it suitable for use in some specialized industrial processes, including the production of certain types of polymers and coatings.