Folded foliation in a metamorphic rock from near Geirangerfjord, Norway (Photo credit: Wikipedia)
Sandstone Shapes (Photo credit: arbyreed)
English: Close-up of granite in Chennai, India. (Photo credit: Wikipedia)
There are three main types of Rock, Igneous, Sedimentary, and Metamorphic. Before I get around to telling you what those are, first let me tell you what a rock is. In general, people think that a rock is just another word for a mineral. WRONG. A mineral is a solid naturally occuring substance that always has the same amount of the same element, such as quartz is always two parts Oxygen to every one part Silicon, whereas a rock is a combination of different minerals, with no uniform formula. And a stone is a word that refers to rocks, minerals, and gems.
Igneous. Let’s say a volcano erupts, and lava starts oozing over the ground. This can turn into different types of Igneous rock in many ways. If the lava flowed into a pool of water, it would cool super quickly, and turn into a sort of glass that we know as Obsidian, a stone that fractures into very sharp points and so was used by the Indians to make their arrowheads. If the lava was flowing around a lot and was frothy when cooled, you have Pumice, the only stone that can float on water, used by silversmiths for a surface to do their work on because it retains heat.For Granite, the lava cools very slowly, letting the contents crystallize, cementing each other together. A good example of this is Granite (first photo).
Sedimentary. Now, take the granite, and we say it falls from the now-cool volcano into the nearby river. The river tumbles and grinds the Granite into sand, which it deposits. This sand keeps on piling up on itself, gradually compacting itself together into a hard substance. This substance is hard, yet easy to carve and cut through. In fact, in Lamanon, France, (where my grandparents live) there is a mini village carved out of sandstone!
Metamorphic. Meta means change, and Morphe means shape, structure, or form. So Metamorph means change in form. Metamorphic rock is formed when any other rock or combination of minerals are forced underground and compressed with immense pressure and changed, so the rock is basically baked compacted into hard form. Gradually movements in the crust can bring the rock to the surface, where we admire and study it today. In fact, what we know as Marble (not the little glass balls, the stone) is Metamorphosed Limestone, which is Sedimentary.
Now remember what I’ve wrote, memorize it, and go correct whoever calls your Emerald a “pretty rock”. Tell them that an Emerald is actually a mineral, as it has a regular elemental composition, and so is way better than a chunk of different minerals cemented together.