In some quartz, such as amethyst and agate, crystals grow inside a rock. These rocks are known as geodes (JEE-odes). A geode is a round or oval rock that has a hollowed-out space, or cavity, inside it. Within the cavity there is enough room for crystals to grow. Crystals often grow if the right elements are present and there is enough time and room for them to grow.
Crystals or other mineral forms line the cavity. The rock and cavity are usually made from other types of quartz, such as agate.
Most of the big amethyst geodes you might see in a museum were formed by cooling volcanic lava.
If quartz is put under pressure, it develops opposite electrical charges at opposite ends. One end will have a negative electrical charge, and the other end will have a positive charge.
This property makes it easy to run an electrical current through quartz. The current vibrates, or shakes, in an exact rhythm. This makes quartz a great material for many electronic devices such as watches, radio transmitters and computers.
A most useful mineral
Jewelers make gemstones from minerals. They shape them so there are facets (FAS-its), or polished sides, that catch the light and sparkle. Many forms of quartz, such as amethyst, are used in jewelry.
Rock crystal is a clear form of quartz. Fortune-tellers claim they can see the future in crystal balls cut from rock crystal.
Quartz is used in manufacturing glass, ceramics and other materials.