Volcanic Glass

There are three types of rocks on earth. They are Igneous, Sedimentary and Metamorphic rocks. Igneous rocks are formed by the solidification of magma and are further divided into two types, Namely, Plutonic rocks(Intrusive igneous rocks) and Volcanic rocks(Extrusive igneous rocks). Plutonic rocks are the rocks that are solidified inside the earth and are usually crystalline in nature due to the slow cooling process Thus they have a coarse grained crystal structure. Volcanic rocks however solidify muck quicker and therefore often show a very fine grained crystal structure. If the cooling and solidification of the volcanic rocks happen even faster it leaves no time for crystal formation. This makes the volcanic rocks amorphous and therefore have properties of a supercooled liquid. That is, These rocks have a glassy texture and are brittle like glass. They also exhibit a conchoidal fracture just like glass. One such rock is Obsidian. Obisidian looks almost artificial and has a remarkable appearance. They are sometimes used to fashion precision cutting tools such as surgical knives. Obsidian has such an appearance that it is hard to believe that it actually is a rock and that it is formed naturally.. It looks artificial. It is a remarkable product of Earth's natural geology. Here's a picture of Obsidian.