Some tourmaline formed from hot fluids containing boron that were given off by magmas far below the earth’s surface. These fluids traveled up through cracks and other openings in overlying rocks. As the fluids reacted with other elements and compounds, the tourmaline formed. The clear, light-colored varieties of tourmaline are much admired, and they are more widely used as gemstones than are the dark-colored varieties. Some collectors, however, find that the dark-colored Texas tourmalines, when cut and polished, make shiny, attractive gemstones. Some tourmaline is used as grinding material, but no Texas tourmaline is produced for this purpose. Travertine. _See_ Calcite. Uranium Minerals (Carnotite, Uranophane, Pitchblende) In 1945, the world suddenly became aware of the awesome power of atomic energy when the element _uranium_ was used to produce some of the first atomic bombs. Uranium does not occur alone in nature but is found combined with other elements in a number of minerals. All of the uranium minerals are radioactive. The uranium they contain is gradually breaking down and changing into a series of 13 other elements, called _daughter_ elements. Each daughter element breaks down and changes into the next daughter element of the series. While breaking down, these elements give off particles and rays of energy. This energy or _radioactivity_ is made up of what are called alpha particles, beta particles, and gamma rays. You cannot see, hear, taste, smell, or feel them. The alpha and beta particles are weak and do not travel far. The gamma rays, however, can travel farther and can pass through seemingly solid material. Scientists have found that these rays can move through about 1 foot of rock, 2½ feet of water, and several hundred feet of air. Prospectors searching for uranium minerals carry instruments that are able to detect this radioactivity. The uranium itself gives off only alpha particles, but some of its daughter elements give off gamma rays. These daughter elements are normally found with the uranium, and it is their strong gamma rays that the instruments are most apt to detect. [Illustration: A Geiger counter is used to detect radioactivity.] One of the instruments used is the _Geiger counter_. It indicates radioactivity by means of a meter, a flashing light, or a clicking sound, which can be heard through earphones. Another instrument for detecting radioactivity is the _scintillation counter_. It is more sensitive than the Geiger counter and it can detect radioactivity from a greater distance. The scintillation counter can be used from an automobile or an airplane, but the Geiger counter must be quite close to the source of radioactivity to be of use.