Minerals are fundamental components of rocks and have great economic and commercial significance to humans. Therefore, geologists need a set of characteristics to identify and describe what makes an object a mineral versus something synthetic (such as plastic, for example). The criteria that defines matter as a mineral is that it is inorganic, composed of an orderly crystalline structure, solid, naturally occurring, and can be expressed by a chemical formula. Some matter looks very similar to minerals, but does not quite meet the criteria. For example, consider glass and quartz. Both are transparent and can have similar appearance. Glass does not have an orderly internal structure; its silica atoms are randomly arranged. Thus it is not a mineral. Quartz, however, has an organized chemical structure of silica atoms, each surrounded by four oxygen atoms and is a mineral.