TOURMALINE is the name given to a family of minerals that have the same chemical composition. Some varieties are gemstones, others are used in industry. It is used in sounding apparatus and other instruments used to detect and record variations in pressure. Tourmalines are popular gemstones, but their electrical properties also make them ideal for use in precision gauges. FELDSPAT minerals are found in abundance all over the Earth's surface, and have also been found on the moon. They are widely used as industrial abrasives and in the manufacture of porcelain and glass. LEPIDOLITE contains important information about the age of the rock layer, and is an essential source of lithium. Some species of LEPIDOLITE can be cut into decorative shapes. While not strong enough for use in jewelry or ornamentation, they are quite popular with mineral collectors.