How to classify rocks
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You classify rocks by Every rock has characteristics that reflect its process of formation; these are used to determine whether the rock is igneous, sedimentary or metamorphic. For example, a rock with rounded grains cemented together is sedimentary, while one with a strong banding and orientation of minerals is metamorphic. Identification of rock types within the three divisions depends on the nature of the mineral grains, including their composition and size and relationship to surrounding grains; the way the rock occurs; how uniform it is; its hardness; how it reacts with acid; its colour; and the way in which it breaks. Before trying to identify a rock, break a piece off with a rock hammer. This will expose a clean, fresh surface.When a rock is dirty or covered with lichen or moss, many of its characteristics are obscured preventing you from accurately identifying the rock. Identification of beach pebbles that are smoothed and rounded is not considered here. Many pebbles are relatively easy to identify without breaking them, but many are extremely difficult, and breaking them usually destroys their interest and charm.