Rocks are different from minerals in that minerals are single crystalline accumulations of one or more elements, while rocks are solid accumulations of one or more minerals. Rock can either refer to a particular, detached object or, in a general sense, as a synonym for stone, generally referring to a portion of the substance. Individual minerals can be obvious in rocks, as with granite, or microscopic, as with many basalts.
A particular mineral is a continuous solid accumulation of a single element, such as a gold vein, or a number of elements in a regular pattern, such as a quartz crystal. Amorphous solids, including glasses such as obsidian, are not considered minerals because they lack a distinguishing molecular or ionic lattice structure. Since pumice and other volcanic glasses can be rocks, not all rocks are actually composed of minerals. Any homogenous solid with a crystal lattice structure, however, including chunks of ice, are considered minerals.
Rocks often contain many different minerals. Most minerals found on Earth are compounds of silicon, with silicon dioxide, also known as silica, being the most common. The most common form of silica is quartz, but other notable forms are chalcedony and opal. Opal is an amorphous solid, however, and so is not actually considered a mineral.