In biology, minerals are defined as inorganic nutrients (this means they do not contain the element carbon). Common minerals that are used in skin care include potassium, calcium, magnesium, zinc, copper, silica, and selenium. Each of these provides unique properties that help to restore healthy skin. In geology, a mineral is defined as a naturally occurring, inorganic solid with a defined chemical composition and a crystalline form. Examples of these minerals used in skin care include gold, silver, pearl, diamond, amethyst, quartz, and tourmaline. These minerals can function as natural antimicrobials and antioxidants, promote microcirculation, and can even act as semiconductors to increase cellular performance and repair. Many of these minerals also provide purely cosmetic effects, for example, diamond helps provide skin with a brilliant luster.