Mineral classification depends on both chemical properties (what elements or compounds make up the mineral) and physical properties (how the mineral reacts with its environment, density, etc.). The most common classification system is the Dana system, named after geologist James Dwight Dana. The Dana system classifies minerals in nine specific categories:
Each of these categories relates to the state of the mineral; for example, Native Elements are found naturally in pure form, while silicates are a compound of metals, silicon, and oxygen. This separates elements from compounds, and allows classification of odd minerals such as amber.
To further classify minerals, the Dana system counts the ratio of cations to anions (positive and negative ions) in the mineral; next, the mineral is compared to other minerals and two numbers are added to indicate these similarities. These base classifications allow the many compound minerals to be easily placed into a category with similar minerals. For example, Halite is the mineral form of common salt; it is a compound of Sodium and Chlorine, and has the Dana class of 09.01.01.01 - Anhydrous and Hydrated Halides.