Mud turns to shale, sand into sandstone, and coarser pebbles cement together into conglomerates. Chemical weathering changes or liberates minerals in rocks. For example, igneous and metamorphic rocks have abundant feldspar, which chemically weathers into clay minerals, and quartz, which is inert to the chemical attack and freed from the parent rock. Chemical sedimentary rocks are precipitated by chemical reactions, evaporation, or organisms, resulting in gypsum, rock salt, and limestone to name a few. Rock weathers into sedimentary deposits that include eluvial, alluvial, and placer.
Eluvial deposits occur when rocks are weathered and the resistant minerals are freed.
They collect in surface or near surface concentrations. Gold is concentrated as an eluvial deposit, as is gem quality olivine or peridot in New Mexico and Arizona. Eluvial deposits are rare though, and more often the minerals freed from the parent rock and moved away from the source by the agents of erosion, wind, water, and ice. Alluvial or placer deposits are accumulated where the resistant minerals and rock fragments become the sand and gravel of stream beds. The grains become less angular and more rounded the farther they are transported and the minerals with higher density or specific gravity, or gem quality material, settle to the bottom of stream beds and water bodies.