Igneous rock is formed from the cooling of liquid magma or lava. There is a very straightforward relationship between crystal size and cooling time for igneous rocks: crystals that appear smaller in a micrograph came from lava which cooled more quickly (formed faster), while crystals that appear larger in a micrograph cooled more slowly (formed slower). If the rock does not appear to have any crystals, but instead looks smooth or glassy, that rock formed the most quickly- the lava cooling was so fast, that crystals did not even have time to form. In general, rocks with small crystals that formed quickly were probably formed on the earths surface where it is cooler, and conversely, rocks with larger crystals that formed more slowly were likely formed deeper in the earth, where it is warmer and the cooling time is longer.