Explanation:
A mineral is a naturally-occurring substance formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure and specific physical properties. A rock is a naturally occurring aggregate of minerals and/or mineraloids. Rocks do not have a definite chemical composition.
Compisition:
Rocks are generally made of two or more minerals. A main determining factor in the formation of minerals in a rock mass is the chemical composition of the mass, for a certain mineral can be formed only when the necessary elements are present in the rock. Calcite is most common in limestones, as these consist essentially of calcium carbonate; quartz is common in sandstones and in certain igneous rocks like granite which contain a high percentage of silica. Two rock masses may have very much the same bulk composition and yet consist of entirely different assemblages of minerals.
Composition of minerals varies from one mineral to another. Some may have a high content of carbonate while others may have a high content of oxides. Even their physical characteristics may vary. While one may be known for its hardness, another may have a characteristic luster. All these variations also form standards for classifying them in broad categories.
Classification:
Rocks are classified by their mineral and chemical composition, by the texture of the constituent particles and by the processes that formed them. These indicators separate rocks into igneous, sedimentary and metamorphic. Igneous rocks are formed when molten magma cools and are divided into two main categories: plutonic rock and volcanic. Sedimentary rocks are formed by deposition of either clastic sediments, organic matter, or chemical precipitates (evaporites), followed by compaction of the particulate matter and cementation during diagenesis. Metamorphic rocks are formed by subjecting any rock type (including previously-formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed.
There are currently more than 4,000 known minerals, according to the International Mineralogical Association, which is responsible for the approval of and naming of new mineral species found in nature. Of these, perhaps 150 can be called "common," 50 are "occasional," and the rest are "rare" to "extremely rare."
Minerals may be classified according to chemical composition, for example- The silicate class, the carbonate class, element class etc. A mineral can be identified by several physical properties such as crystal structure, hardness, color, luster, specific gravity etc.
Valuable:
Some minerals are valuable due to their use as an important ore or for their use as a gemstone. For example, the silver ores may not necessarily look appealing, but demand a high price because of their chemical composition. Some minerals such as the rare Phosphophyllite have become icons in the mineral trade, and are due to their rarity and insatiable demand command outrageous prices.
Rare minerals will be more valuable than more common ones. Fairly common minerals, even if nicely colored and well formed, can be very affordable because of their commonness (unless they are exceptional in other regards).
