All materials are categorized as either organic or inorganic. Organic materials are made from (or extracted from) plants or animals. Inorganic materials are made from rocks and minerals.
Organic materials include wood, paper, textiles, and animal parts (ivory, bone, leather). They also include some natural materials that are not obviously organic (such as coal and fuel oil, which are derived from fossilized plants and animals) and some synthetic materials (such as plastics, which are made from chemicals extracted from plant and animal products). All organic materials deteriorate over time.
Inorganic materials include stone, metal, ceramic, and glass, which are all made from rocks or minerals. Some inorganic materials are found in paper-based formats: photographs contain metallic particles; some pigments and inks contain minerals, metals, or metallic oxides; and metal particles are sometimes found in paper itself. Inorganic materials are generally stable individually, but they can react with other materials to cause deterioration. Some metals (particularly iron, copper, and platinum) react with the cellulose in paper. Like organic materials, inorganic materials can be natural or synthetic (e.g., some pigments occur naturally as minerals but can also be manufactured from other inorganic materials).