1 It is proven that a cup of hot coffee can help uplift your senses and refresh you. This quality of coffee can be attributed to the presence of caffeine, which is a central nervous stimulant.  Caffeine is basically a bitter white crystalline alkaloid.
1 Igneous rocks that form too quickly for crystals to form have what is called a glassy texture. 2  Obsidian is the only glassy igneous rock, and can be identified by its dark color. 3  This looks like dark black glass. An example of an intrusive rock is granite. 2  When magma is above the earth's crust, it is known as lava. 3  Extrusive rocks form through very fast cooling of lava above the Earth's surface. 4  Extrusive rocks have very small, almost microscopic crystals.
Extrusive igneous rocks are formed on the surface of the earth. These rocks are formed as a result of a volcanic eruption. Most have a fine grained texture made of small crystals due to rapid cooling. However, some of the lava cools so quickly that no crystals have time to form. An example of this type of rock is obsidian, commonly called black glass or volcanic glass.