Why are minerals found in certain locations?

In the very thin shell of the Earth's crust are found many minerals. Because of their purity, scarcity, or social value, some minerals are called gems.

Minerals share five common characteristics:

They occur naturally in the earth or in other words the earth makes them.
They are inorganic meaning they have never been living.
They have a definite chemical composition.
They are always found as a solid.

The atoms of the mineral are arranged in a definite, repeating pattern, which is called a crystal.
The earth uses gravity to generate heat and pressure which sorts minerals by density. Later, chemical bonds form which also create different densities of rocks. Gravity then does more sorting by pulling the more dense objects closer to the earth's core.

Volcanoes are nature's method of naturally refining and sorting minerals. Precious minerals such as copper, silver, gold, and iron are in solution within the magma, which is deep within the earth. In solution, these minerals are in such low concentrations that they cannot be mined unless the magma is to harden.

Water now becomes very important in the smelting process within the earth. Many of the most important mines in the early history of civilization such as Cyprus, relied upon copper for tools. Surface water and precipitation naturally seep into the earth's crust as the magma cools and openings form. The cooling and condensing of the magma create the cracks, fissures, and vents produced by the seeping water. This can occur in a surface volcano or an underwater "smoker". The water bonds to the copper in solution, and then carries the precious mineral back up the cracks, fissures or vents where it then hardens. The mineral is now in a higher concentration than it was in the magma.

Mineral deposits are concentrations of minerals and, as such, is a geological term. Whether a mineral deposit is also an ore deposit depends on economics. "Ore" is therefore an economic term.

Ore deposits can be classified according to a number of different criteria, such as

the minerals contained within the deposit (e.g. porphyry copper deposits)
the shape or size of the deposit (e.g. strataform and stratabound deposits)
the host rocks (e.g. shale hosted deposits, breccia pipes)
the genesis of the deposit (see below)
There are basically five genetic processes that lead to the concentration of minerals

Hydrothermal mineral deposits form in association with magma and water;
Magmatic mineral deposits concentrated in igneous rocks;
Sedimentary mineral deposits are precipitated from a solution, typically sea water;
Placer minerals are sorted and distributed by flow of water (or ice);
Residual mineral deposits formed by weathering reactions at the earth's surface.