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Igneous rocks form when magma cools. But some magma cools far underground, and it cools slowly. Usually that means individual crystals have time to grow, and the resultant rock is coarse-grained. That means you can see the individual mineral grains without a hand lens.
Igneous rocks that cool far below ground are called plutonic, or intrusive. We wouldn't find them at the surface at all except that mountain-building processes raise them up, and erosion clears away everything that was on top, leaving the once-buried rock exposed to the atmosphere - and to rockhounds!
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