As already mentioned, most rocks contain crystals. Crystallography can be used to help classify rocks by identifying the crystals that they contain. The scientists can work out the chemical structure of the crystals, and therefore the environment in which they were formed. If the crystals are large, they have obviously had a long time and a lot of space in which to grow. If the crystals are smaller or imperfect, they formed quickly or perhaps in a limited space.
Crystals can also help when determining the age of rocks.
This makes it possible to determine the age of the rock from the age of the grains. There are other radiometric dating methods available for larger samples of rock. Geodes Geodes are rocks which have a hollow space inside them where crystals have grown. Hot silica-rich waters have filled the spaces in the rocks, and over thousands of years these waters have evaporated to leave crystals behind. Geodes are also called eagle eggs because some ancients believed that eagles placed the stones in their nests to protect their eggs.