Sedimentary rocks are rocks formed from fragments of other rocks as well as organic materials such as shells. There are multiple different groups of sedimentary rocks. They can be classified as clastic, which is rock fragments put together because of massive amounts of pressure, chemical, which occurs when water evaporates, which leaves behind previously dissolved minerals, and organic, which is when sediments are assembled though natural processes.
Chemical Sedimentary Rock
Sedimentary Rocks are found in places such as rivers, oceans, and deserts.
Igneous rocks are rocks which form from molten materials. The word Igneous is taken from the Latin term "Ignis", which means fire. Igneous rocks form in two ways. They can form intrusively, which is when they cool slowly under the ground, developing large crystals and gas pockets. On the other hand, they can cool extrusively, which is when they cool rapidly above ground, having no time to develop crystals or gas pockets.
Igneous rocks are found in or near volcanoes.
Metamorphic rocks form deep within the ground. Initially, they start life as a sedimentary rock. Heat and pressure begin to change the rock. The various rocks begin to sort themselves by density, creating bands on the rock. That is one of the main identifying features of a metamorphic rock.
Gneiss is a metamorphic rock.
Tools for Identifying a Rock
When trying to determine the identity of a rock, there are multiple different tools that you can use to determine the identity. You can use a streak plate, which will give you an accurate color of the rock in its powdered form.
Another tool to determine the identity of a mineral is hydrochloric acid. It is used to determine if a rock is made of calcite, which bubbles in contact with hydrochloric acid.
Also, you can run a mineral across a piece of glass to determine its hardness.