Igneous rocks

Igneous rocks consist of material that cooled from a molten state.

Telltale signs often include glassy textures and/or well-formed crystals embedded (not just stuck in). Some igneous rocks that cool very rapidly can have holes -- vesicles -- in them.

http://www.kidsgeo.com/geology-for-kids/0026-igneous-rocks.php

Below are granite (left) and obsidian (right), both igneous rocks. Granite is an intrusive rock, which means that the magma cooled underground. Obsidian is an extrusive rock, which means it cooled above the Earth's surface. In general, larger crystals means slower cooling time (often corresponding to underground, as opposed to above ground).

http://www.scienceclarified.com/Ro-Sp/Rocks.html

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