There are even huge ones like so:
Geodes range in size and can be filled with different types of minerals/crystals such as amethyst, agate, quartz, calcite, celestite, etc.
Here are my amethyst pieces which came from the inside of a geode like the one above. Plus a little pendant made of amethyst.
Calcite is the most stable polymorph of calcium carbonate (sorry I love chemistry), they are usually white, green, or yellow. Calcite has a lot of optical uses and has also been employed in experiments involving the construction of an invisibility cloak.
You might recognize hematite from jewelry, particularly beads. I'm pretty sure the second one is chalcopyrite but I'm not positive, and pyrite is silverish-gold with little cubes as you can see. During the gold rush people would often mistake pyrite for gold and still do.
Turquoise mineral is really popular in jewelry making because of it's color. Quartz comes in a lot of different colors such as white and soft pink which is called rose quartz. The heart pendant on the right is made of quartz.
After I took the picture I found out that it's bornite, which is also known as peacock copper, cool!