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Instructions
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Identify the type or types of minerals in the rock. The list of minerals that forms all rocks is short. Minerals such as calcite dissolve if they come into contact with hydrochloric acid. Quartz, for example, has a glassy texture and a waxy, smooth look. The mineral quartz sand produces sandstone as well as the metamorphic rock quartzite.
Determine if the rock reacts with hydrochloric acid. If it does, you know the rock contains calcite. Calcite does not form igneous rocks, so you know the rock is either a sedimentary rock or a metamorphic rock.
Determine if the rock has any banding. Banding is a process formed by extreme heat and pressure whereby certain deposits migrate to form alternating light and dark colored bands in the rock to produce a new foliation. Rocks that exhibit banding are nearly always metamorphic. Recognizing the position of the minerals in metamorphic rocks is often the most important clue in determining what type of rock it is.
Determine how the minerals fit together in the rock. When the minerals have no certain order of arrangement, the rock is likely a sedimentary rock. When the minerals fit together like pieces of a jigsaw puzzle, the rock is likely an igneous rock. When the minerals are layered or banded in the rock, the rock is likely a metamorphic rock.
Look to see if the rock has holes or bubbles. If it does, you know the rock is an igneous rock. Igneous rocks like pumice have many tiny holes formed by the extreme heat of the gases in the Earth's crust, while igneous rocks like scoria have very large holes that almost look like small pockets in the rock.