Pyrite is a shiny, pale golden-yellow or brassy-yellow metallic mineral. This mineral, an iron disulfide, is so often mistaken for gold that it is widely known by the nickname _fool’s gold_. Except for their similar color and luster, pyrite and gold are really very different. When you rub pyrite across a streak plate, it leaves a black, a greenish-black, or a brownish-black streak, but the streak of gold is gold-colored. Pyrite is too hard for the average pocket knife to scratch, but a knife will scratch gold easily. Pyrite is brittle and readily breaks, but gold is malleable and flattens out when hit with a hammer. Pyrite is only about 5 times as heavy as an equal volume of water, but pure gold is over 19 times as heavy. And pyrite may have a brown or a multicolored tarnish on it, but gold never tarnishes. [Illustration: Pyrite veins in white marble from Llano County, Texas.] [Illustration: Cubic crystals of pyrite.] Pyrite is a common mineral and is found in many of the igneous, metamorphic, and sedimentary rocks of Texas. It may be scattered through the rocks, or it may fill cracks and cavities in them. This mineral occurs as granular and compact masses, as rounded masses, or as crystals. The crystals are commonly cubes, pyritohedrons, or octahedrons. In some crystals, the shapes are combined (such as a cube with an octahedron or two pyritohedrons grown through each other). You may notice that the sides of some cubes and pyritohedrons have fine, parallel grooves (called _striae_ or _striations_) on them. Pyrite originates in a number of different ways. Some of it forms, along with other minerals in igneous rocks, from hot magmas. It also forms in metamorphic rocks by the same processes that produce these rocks. Some of the pyrite in limestone and other sedimentary rocks is formed when the rocks themselves are deposited by seas or streams. Pyrite also is deposited by the hot fluids that are given off by magmas. These fluids travel up into cracks and other openings in rocks and then form pyrite as well as other minerals. Much pyrite forms in still another way. As water seeps through rocks, it dissolves some of the iron minerals that they contain. When, under certain conditions, these iron solutions mix with hydrogen sulfide (this is the gas that makes some water smell like rotten eggs), pyrite is formed. Pyrite alters easily. Because of this, most builders carefully check the limestone, granite, marble, or whatever other building stone they plan to use to be sure that it does not contain large amounts of pyrite. When exposed to the weather, pyrite changes to limonite and causes an unsightly rust stain. Pyrite is used as a source of sulfur, and it is produced for this purpose in several states. In Texas, however, no pyrite deposits have been found that are large enough to be mined. Pyrolusite. _See_ Manganese Minerals. Quartz