Hobbies And Interests
Roundness
Geodes are generally round in structure. The most common ones people see are somewhat spherical, but they can also be tubular or bulbuous. The roundness suggests that geodes can be formed as a result of gas bubbles, such as those formed through lava flows. The rock around the bubble cools and hardens over time, forming the other shell of the geode. Other possible ways a hollow can be formed is through animal burrows, tree roots that have decomposed or just natural fissures in the earth.
Crystalline Interior
What makes a geode a prized find is the crystalline structure in its interior. There crystals can be composed of many kinds of minerals, but calcite, amethyst and quartz are the most common.
It's not possible to tell from the outside shell of the geode exactly what the interior is going to look like; one would have to crack the geode open to see what colors and crystal formations have resulted over the thousands of years of silica precipitation that have deposited layers of crystal inside the stone. Different colors result from the various impurities within the crystals.
However, geodes from the same location are likely to have the same mineral deposits, and thus the same colors and structural qualities. Still, avid geode collectors angle for rare types of geodes, like those composed of rare amethyst or black calcite.
Hollow Core
One main characteristic of a geode is the hollowness at its core. It is possible for mineral deposits to completely fill a hollow with crystals. Those structures are called nodules.
The world's largest geode is located in Put-in-Bay, Ohio. It's called Crystal Cave and contains enormous bluish celestite crystals--up to18 inches long--which were originally mined for fireworks. Nowadays the cave is used for tours operated by the family-run winery that owns it.