Igneous rocks.

Igneous rocks start out as molten rock — magma below the surface, or lava once it reaches the surface — and form when that melted rock cools and hardens into solid crystals. The single biggest thing that determines what an igneous rock looks like isn't what it's made of, it's how fast it cooled.

Magma trapped deep underground cools slowly, sometimes over thousands of years. That slow cooling gives minerals time to grow into large, visible crystals — these are called intrusive (or plutonic) rocks. Granite is the classic example: look closely and you can actually see the individual mineral grains.

Lava that erupts and cools at the surface, by contrast, cools fast — sometimes in minutes. There's no time for large crystals to form, so these extrusive (or volcanic) rocks are fine-grained, sometimes so fine you can't see individual crystals at all. Basalt is the most common example on Earth — most of the ocean floor is basalt.

Same basic ingredients, wildly different rock, just from cooling speed. That's the thing worth remembering about igneous rock.