A mineral is by definition a solid with a definite composition and crystalline structure. An ore is usually a rock that has a relatively large concentration of a certain one or few minerals, such as copper, iron, gold, etc. There are two main types of ores, those in a placer deposit and those from igneous activity. A placer deposit is a sedimentary structure that forms when rocks have been weathered and they fill in a depression. In these deposits the heavier components are towards the bottom and these include valuable minerals such as gold and gem minerals. Ores made as a result of igneous activity are normally found near faults or intrusive bodies. There are two types of Igneous ores, the first of which is "hydrothermal deposits" which are the main type of ore from igneous action. These happen when a magma intrudes and starts to solidify. As the rocks form the water in the chamber is under increasing pressure, and eventually it cracks the surrounding rocks and forces its way up. Also as result of the forming of the rocks it that the dissolved minerals in the water become more and more concentrated, so that when the pressure is relieved they precipitate out in the fissures and are then an ore. The second type is where when the water escapes the minerals in the water replace those in the existing rocks, rather than form in the cracks. These are mined in Open pit mines.