A Metal, in Graettinger, can be defined as an element or an alloy which is a very good conductor of heat and electricity. Metals usually react with oxygen present in the air, and form oxide. Examples of metals are nickel, copper, iron, zinc, gold, platinum ,etc. Corrosion of these metals can be avoided by plaiting the metals, anodizing or by painting them. The luster and density of the metals are usually high. Since light waves are not easily transmitted through metals, they are lustrous, opaque and shiny. Due to inherited capacity for plastic deformation, metals have an important feature and that is ductility. Metal plays a very important role in astronomy for the very reason that it refers to all elements; however, these elements do not include hydrogen. According to the science of chemistry, metals do not hold any relevance in astrology. Metals can be classified as base metal, precious metal, ferrous metal; 'base metal' is defined as a metal that corrodes or oxidizes very easily; copper is the best example of a base metal. An important property of ferrous metals is that they are magnetic; wrought iron is an example of ferrous metals. Rare metals are precious metals, such as platinum, gold, etc.