Igneous rocks

1.Basic
     1.1.Volcanic or extrusive rock
     1.2.Plutonic or intrusive rock

2.Advanced
     2.1.Stages of the igneous or magmatic cycle

3.Expert
     3.1.Zircon to calculate the age of the earth

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Basic

By the cooling down of magma, atoms are linked into crystalline patterns and subsequently different minerals are formed. When the formation takes place in the depths of the earth's crust (approx.33km deep) quite large rocks may be formed (for instance, granites).

Igneous rocks are formed and created by magmatic processes in the earth. To form very large crystals of rare minerals, exceptional conditions are needed. For instance, a rock called pegmatite is formed by the crystallization of magma enriched with water in the veins of other rocks, and may contain beryl, tourmaline and topaz.

Igneous rocks are divided into two types-volcanic rock (extrusive) and plutonic rock (intrusive)-depending on where the magma cools.

Volcanic or extrusive rock

This is rock that is formed on the surface of the earth. In contact with air or seawater, molten rock cools rapidly and either quenches to a glass (like obsidian) or forms small crystals (basalt). Volcanic rocks are usually finely grained or glassy in structure.

Basalt is an extrusive rock, finely grained due to its rapid cooling. It largely consists of tiny feldspar and pyroxene crystals (like diopside and enstatite). Some basalts contain gemstones like corundum, zircon and garnets.

Another volcanic rock is called kimberlite. Kimberlite pipes are the most major source of diamond.

Occasionally, varieties of volcanic glass, obsidian, are cut and fashioned as gemstones. Obsidian is an amorphous mineraloid with the hardness of approximately 5.5. Varieties of obsidian include:

Snowflake obsidian (with inclusions of the mineral cristobalite)

Rainbow obsidian

Red mahogany obsidian

Silver sheen obsidian

Midnight lace obsidian

Pumpkin obsidian

Apache tears obsidian

Plutonic or intrusive rock

When molten rock solidifies within preexisting rock, it cools slowly, forming plutonic rocks with larger crystals. They tend to be coarse grained.

Granite is a coarse grained intrusive rock which contains the minerals quartz and feldspar, and usually carries mica or hornblende. In some circumstances, granite undergoes "fractional crystallization", a process where slow cooling creates crystals of different minerals as they form at different temperatures.

Minerals of the pegmatite group are among the last to be formed, often occuring as veins penetrating their surroundings.

Associated minerals that find their origin in igneous rocks

Beryl

Chrysoberyl

Corundum

Diamond

Garnet

Feldspar

Peridot

Quartz

Spinel

Topaz

Tourmaline

Zircon